Why are certain non-harmful behaviors considered sins? I would like to hear your response on this. In general, there exist a set of behaviors that Christians consider sinful, though these behaviors may not be observed to harm anybody (even the person doing the behavior.) 

Why might this be? 

I am assuming that God is not simply making rules for the heck of it, but that there must be some consequence of some kind by which we may understand why such a behavior might be considered wrong or sinful.
The reason that something may be considered a sin primarily because it hurts your relationship with God. When God has ordered the world in a certain way, or decrees that certain things are not to His will, then going against it is a sin. A sin isn't something that is bad for others. It's bad for ourselves. So to judge something as sinful because it hurts others isn't going to get the full picture (though it certainly is one aspect of a sin, as we are also called to love others, by God).

However, there were certain laws that the Jews were told to keep in the Old Testament. As you mentioned Kosher dietary restrictions, I'll use that as an example. The Jewish people at the time were very much in danger of being annihilated by the various nations in the time. They were constantly at war, and they were nomadic, living simply off the land. God desired for them to flourish and prosper, and so certain laws were decreed that ultimately had the result of encouraging good health and hygiene. Kosher laws at the time were very useful in preventing food-born disease and healthy eating habits. Other things, like purification rituals, have a direct correlation to the modern guidance by our parents to wash our hands before we eat. You can imagine that at that time, it was even more important.

There are different kinds of decrees in the Bible. There are things considered outright sins, but also laws and regulations. Many of these (that aren't actually sins) were for certain peoples at certain times, but ultimately a sin is when we go against the will of God, through our own free will.

A good read here: http://www.newadvent.org/cathen/14004b.htm