Why don't priests give Holy Communion to everyone at Mass? Today I went to a Catholic Church and attended the mass then at communion time, priest asked us all to receive Communion (white, rounded hosts), but when I went there he spoke something very slowly and I did not listen properly, I asked him to repeat he did not repeat and was told that I could not receive Communion.

Please explain the case. Was the priest rude, or is there any special rule in Catholic Church for receiving Communion?
Why don't priests give bread to everyone at mass? 

Priests are normally on their toes at all Masses on Christmas Eve and Christmas Day. The reason is quite simply. Many non-Catholics show up and many Catholics who are not properly disposed to receive Communion show up. In order to receive Communion in a Catholic Church, one must be Catholic and a practicing Catholic at that. In other words, a Catholic must be in the state of grace before receiving Holy Communion.

It has been decades now that I have noticed that priests have made an announcement just before the reception of Communion of the requirements needed to receive Our Lord in the Sacred Host. Priests are not being rude in doing so, they are simply, a matter of fact like, doing their duty so to speak!

One a simple note, Catholic must fast for one hour before receiving Holy Communion and more follows.

What does Canon Law say in this matter?


  Can.  919 §1. A person who is to receive the Most Holy Eucharist is to abstain for at least one hour before holy communion from any food and drink, except for only water and medicine.
  
  Can.  915 Those who have been excommunicated or interdicted after the imposition or declaration of the penalty and others obstinately persevering in manifest grave sin are not to be admitted to holy communion.
  
  Can.  914 It is primarily the duty of parents and those who take the place of parents, as well as the duty of pastors, to take care that children who have reached the use of reason are prepared properly and, after they have made sacramental confession, are refreshed with this divine food as soon as possible. It is for the pastor to exercise vigilance so that children who have not attained the use of reason or whom he judges are not sufficiently disposed do not approach holy communion.


Non-Catholics Receiving Communion


  The guidelines for receiving Communion, which are issued by the U.S. bishops and published in many missalettes, explain, “Because Catholics believe that the celebration of the Eucharist is a sign of the reality of the oneness of faith, life, and worship, members of those churches with whom we are not yet fully united are ordinarily not admitted to Communion. Eucharistic sharing in exceptional circumstances by other Christians requires permission according to the directives of the diocesan bishop and the provisions of canon law.”
  
  Scripture is clear that partaking of the Eucharist is among the highest signs of Christian unity: “Because there is one bread, we who are many are one body, for we all partake of the one bread” (1 Cor. 10:17). For this reason, it is normally impossible for non-Catholic Christians to receive Holy Communion, for to do so would be to proclaim a unity to exist that, regrettably, does not.
  
  Another reason that many non-Catholics may not ordinarily receive Communion is for their own protection, since many reject the doctrine of the Real Presence of Christ in the Eucharist. Scripture warns that it is very dangerous for one not believing in the Real Presence to receive Communion: “For any one who eats and drinks without discerning the body eats and drinks judgment upon himself” (1 Cor. 11:29).


Addendum:

At one time, it was somewhat common practice to take home some ”Blessed Bread”. These blessed breads were generally associated with a  particular diocese or saints.


  The little loaves or cakes of bread which received a special benediction and were then sent by bishops and priests to others, as gifts in sign of fraternal affection and ecclesiastical communion were also called eulogiae. Persons to whom the eulogia was refused were considered outside the communion of the faithful, and thus bishops sometimes sent it to an excommunicated person to indicate that the censure had been removed. Later, when the faithful no longer furnished the altar-bread, a custom arose of bringing bread to the church for the special purpose of having it blessed and distributed among those present as token of mutual love and union, and this custom still exists in the Western Church, especially in France. This blessed bread was called panis benedictus, panis lustratus, panis lustralis, and is now known in France as pain bénit. It differs from the eulogia mentioned above, because it is not a part of the oblation from which the particle to be consecrated in the Mass is selected, but rather is common bread which receives a special benediction. In many places it is the custom for each family in turn to present the bread on Sundays and feast days, while in other places only the wealthier families furnish it. Generally the bread is presented with some solemnity at the Offertory of the parochial Mass, and the priest blesses it before the Oblation of the Host and Chalice, but different customs exist in different dioceses. The prayer ordinarily used for the blessing is the first or second: benedictio panis printed in the Roman missal and ritual. The faithful were exhorted to partake of it in the church, but frequently it was carried home. This blessed bread is a sacramental, which should excite Christians to practice especially the virtues of charity, and unity of spirit, and which brings blessings to those who partake of it with due devotion. The Church, when blessing it, prays that those who eat it may receive health both of soul and body: "ut omnes ex eo gustantes inde corporis et animae percipant sanitatem"; "ut sit omnibus sumentibus salus mentis et corporis". In some instances the pain bénit was used not only with superstitious intent, and its virtues exaggerated beyond measure, but also for profane purposes. This usage was brought from France to Canada, and was practised chiefly in the province of Quebec. There the pain bénit had blessed immediately after the Asperges, and then distributed to those who assisted at high Mass. The parishioners furnished it in turn, and vied with one another in presenting as rich and fine a pain bénit as possible, until finally the bishops, seeing that it entailed too much expense upon the poor circumstances, prohibited it. Within the last twenty-five and thirty years the custom has almost entirely disappeared.
  
  In the present Roman ritual there are six blessings for bread. Two of these are entitled simply benedictio panis, and as mentioned above, are often used for blessing the panis benit. The third entitled benedictio panis et placentarum (blessing of bread and cakes), is found in the appendix among the blessings which are not reserved. The other three are approved for particular localities, and are special blessings given under the invocation of certain saints, usually on their feast days, in order to gain special favours through their intercession. The first, approved for the Archdiocese of Cologne, is a blessing of bread, water, and salt given under the invocation of St. Hubert; the second, approved for the Diocese of Bois-le-Duc, is a blessing of bread and water under the invocation of St. Machutus, and the third, for the Diocese of Urgel, is a blessing of bread, wine, water, and fruit to be used on the feast of St. Blasius. Some other places have local of blessing bread on certain feast days, as for instance on the feasts of St. Genevieve, of St. Nicholas of Tolentino and others. - Liturgical Use of Bread