What does Paul mean by "Love believes all things?" In 1 Corinthians Paul says 


  Love bears all things, believes all things, hopes all things, endures all things. (1 Corinthians 13:7, ESV)


This has long puzzled me from a couple of angles.  

First, is this instructive, or descriptive?  What I mean is, is Paul saying that to be loving, this is how we must behave?  Or is he saying, someone who loves just naturally displays these characteristics?

Second, if this is instructive, what does it mean that love believes all things?  Hopes, and bears/endures, I get, but believes?  Let me use a personal example: an adulterous spouse.  I love her and she claims with great energy that it will never happen again.  I believe her, but it does.  And she claims again that it will stop.  I believe her??  Every time?  For how long?  

If it is descriptive, then believes makes sense, because as the cliche says, "Love is blind" so of course love believes all things.  

Any insights into this?
Since Paul didn't expand on this, the best we can do is to review what noted theologians have said about this.  To get some good answers, you really need to look no further than Bible commentaries.   

Bear in mind that the type of love here is agape love, which is also translated as charity, or selfless love.  It's not speaking of romantic love as on "love is blind", it's speaking of the selfless love that puts others first.

The general consensus is that it means that if you are truly charitable, you will have the type of love trusts by default.  That you're not cynical by nature.

To back this up:

From the Pulpit Commentary


  Believeth all things. Takes the best and kindest views of all men and
  all circumstances, as long as it is possible to do so. It is the
  opposite to the common spirit, which drags everything in deteriorem
  partem, paints it in the darkest colours, and makes the worst of it.
  Love is entirely alien from the spirit of the cynic, the pessimist,
  the ecclesiastical rival, the anonymous slanderer, the secret
  detractor.


From Gill's Exposition of the Entire Bible


  believeth all things; that are to be believed, all that God says in
  his word, all his truths, and all his promises; and even sometimes in
  hope against hope, as Abraham did, relying upon the power,
  faithfulness, and other perfections of God; though such a man will not
  believe every spirit, every preacher and teacher, nor any but such as
  agree with the Scriptures of truth, the standard of faith and
  practice; nor will he believe every word of man, which is the
  character of a weak and foolish man; indeed, a man of charity or love
  is willing to believe all the good things reported of men; he is very
  credulous of such things, and is unwilling to believe ill reports of
  persons, or any ill of men; unless it is open and glaring, and is well
  supported, and there is full evidence of it; he is very incredulous in
  this respect:


Jamieson-Fausset-Brown Bible Commentary


  believeth all things—unsuspiciously believes all that is not palpably
  false, all that it can with a good conscience believe to the credit of
  another. Compare Jas 3:17, "easy to be entreated"; Greek, "easily
  persuaded."