I keep hearing from people that pairs (couples) are made in heaven, and just celebrated on earth. This, to me, implies that basically it has already been decided as to who one will marry. Could you kindly throw some light on it?

What it means is that every decision that is taken on earth is already known to the Almighty beforehand, because He is the All-Knowing God. However, in the case of some such decisions, what is known to Him is actually expedited through us, as we use our discretionary powers. In other words, he knows beforehand what we will decide even though His foreknowledge doesn’t interfere in our free-decision making process. I believe that decisions on marriages, in most cases, belong to this category.

However, there are other decisions known to the Almighty which He Himself has taken, for which our discretion is not allowed to play any role. Our birth, death, parents, and relatives etc., are some of the things on which we have not been given any discretion. But wherever there’s room to act, we must act and consciously deal with a situation as best as we can.

Courtesy: http://www.islamicissues.info/qa_question.php?qid=538