there is no such thing as "falling in love". it's not some cosmic thing.
there is attraction, and then there is attachment. you can't choose whether you're attracted or not, that's involuntary and happens whether you're single or not. but you can choose whether to pursue the attraction to the extent that you form an attachment.
(btw the attachment can be healthy or unhealthy. the fact that you are attached doesn't mean you should be attached/that it's a good idea to be/that it's "meant to be". you can be attracted and/or attached to a complete knobber, as many MN threads will indicate)
when you are married or involved, the idea is to see the attraction for what it is and involuntary, hormonal, almost animal-like sort of thing and avoid feeding it with behaviours like talking to the person, flirting, trying to be near the person, etc.
when you are single, of course you're free to pursue the attraction, and therefore to form an attachment if you so wish.
it's not magic. people are not "meant to be together", and therefore when you have feelings for someone, it doesn't mean you're meant to pursue them.
we're conscious animals, we're responsible for actually thinking about our emotions/attractions before deciding on how to respond to them.