Love makes you do the wacky. I don't think the belief that you will never loved another as you love a certain person makes a love immature. No matter how mature you are, when you fall in love you feel that. You think that no one has ever felt the way you feel now. You can be 15 or 50 when that feeling overtakes you. Spike makes proclaimation like this about Buffy yet he is never described as childish or immature, in fact he is seen as hopelessly romantic.
It works both ways folks.
Meh, differing opinions. Will take your comment on Spike, though, touché. Although I don't remember him saying he could never love anything else as much, which is where I find my problem with Buffy's statement. That's saying your friends, your sister, your... whatever Spike was, could never mean as much as that one person. Just don't agree with it.