Only you have the power to allow yourself to fall in love: no one else. I've always said (and I get older, it becomes ever-more apparent) that love is the gift you give yourself.
I've had my fair share of fierce crushes. Some I've allowed myself, others I just nipped in the bud and moved on. It's striking to me that these have little to do with the other guy. Less frequently, I'll allow myself to be the subject of someone else's infatuation: if I cannot honestly return the feelings in kind, I rarely see any reason to encourage anything.
But love, real heart-crushing, soul-minimizing, all-consuming, stand-in-the-rain-without-an-umbrella love has really only happened to me three times. It's an ego-shattering, head-long dive into emotion and passion. I've been lucky enough to have it returned twice. The third time was like firing a blank, though it didn't feel any different, really. And even though the two who did return my love in kind have been dead for many years now, I'll never love them less, and their lack has left holes in my soul that will never really be healed.
In between what I've described above and a crush is what I call soft-contentment love. This I've felt more often. In fact, my two longest relationships (four and nine years, respectively) were just of this type. I fell in love with them completely, loved them deeply, and tried with all my heart to make the commitment work. I don't look at the fact that they eventually ended as any failing, just a facet of the limitations of the guys involved. If things had worked out differently, I'm sure we'd still be together. And I have the satisfaction of knowing that I was not the one who couldn't live up to the lives we tried to share.
The minute I'm no longer capable of giving and receiving love is the moment I hope I never live to see, as I'll be so fundamentally altered so as to be unfamiliar to myself and those who know me.