I think we may feel love for someone, or for some thing, based on how much effort we have invested. Now, I don't mean the sort of trust, admiration, and respect of a more intellectual love, but am speaking more of the gut wrenching, spine tingling, rip your heart out variety. This would explain why it always seems to be the person who puts in the most effort that falls the hardest; which is very unfair, and doesn't make a lot of logical sense, but often the way it goes.
There is of course the problem that, though there is obviously a strong
correlation between love and effort, it would be difficult to establish "effort"
as bringing about "love" -- especially since it's commonly thought that the
cause and effect relationship is the reverse.