If a person is really in love with, and loves, another person; there's never any doubt whatsoever when it comes to "deciding".
There can be "liking/caring about someone else as a friend".
Then there can be "finding someone else an attractive person" (and liking him as a friend, not liking him at all, or not caring one way or another about him).
There can be
There isn't, however, having trouble "deciding". When there's "trouble deciding" what the person has for one and/or both other people either is not real love, is not "complete" and genuine love, or else is an immature form of love. "Immature" can happen for people of all ages; but if someone is young "immature" isn't at all an insult. Part of being young is being "not-mature". Since you've said you've seen it in "so many cases", I'm guessing you may be young and hanging out in young circles where you're seeing all these cases. (I could, of course, be wrong with my guess, needless to say. :) )
So, my answer to "What it is it?" is that I suspect IT is a matter of being young and not yet knowing what loving someone in a whole and mature way really feels like. Young people (and older ones who aren't completely emotionally mature) tend to look a potential partners the way the proverbial kid-in-a-candy store does. They look at what what looks most appealing, have trouble deciding which they want, and think in terms of what life would be like if they had "that one".
When love is mature and whole there's no "candy store" window to be seen because we own the candy store, along with every other kind of "store" one could ever imagine. :)