They call it "falling in love" because that's what happens: you fall in love with little or no prior warning, as if it were a tiger pit in the middle of an Indian forest.
They don't call it "easing in to love," or "walking in love," or "jumping in love," or "wading in love," or "running in love" or even "diving in love." All of those verbs imply some force of will, some sense that you intended this to happen and in fact moved toward it. Which is not to say that no one ever intends to be in love. Almost everyone does, and in fact we do say that someone is waiting for love, but the truth is when it comes it catches us by surprise, even if we thought we saw it coming.
Love is more like a black hole, pulling us in with an almost inescapable force somewhat akin to gravity and just as natural. Oh, sure, one can try to resist, and many do: clinging to anything they can as they skid their heels along the ground, ending up bruised and battered but out of love just in the nick of time. Once you've passed through the event horizon, though, there's no turning back. You are doomed to be drawn into and enveloped by love, which slowly wraps itself around you and begins to squeeze much as a boa constrictor does its prey. It overtakes you, it consumes you, it constricts your brain so that you can think of nothing else.
And just before you lose consciousness, you find yourself thinking that you've never felt so absolutely incredible in your entire life.