To love is to be vulnerable, yes. And to be vulnerable is to be released from the pressure that comes with having to be anything other than our real and actual selves. To be vulnerable is to be essentially ourselves, to the fullest extent, even if we're not sure exactly who we are yet. Maybe this is what people mean when they refer to the love of another strengthening them, making them better- they mean that they are totally free, to be exactly who they are, and more. That, I suppose, is reason enough to want to love. That kind of freedom is also reason enough to be scared to death, which is perhaps why some people lock themselves up in their selfishness, as Lewis says. To be vulnerable isn't easy, and maybe the instant in which it happens with another, it has to mean, in some form, love. Maybe it really is as simple and complicated as that.
Fill up your cup.