Answer:
well to put it in simple form,
spike started having dreams about Buffy,but not ordinary dreams i mean real hot dreams and then he started being nice to her and started falling in love with her for real and then Buffy started falling in love with him and blah, blah,blah......
and then they eventually become something like boyfriend and girlfriend.
so there you go that,in my version of simplest form, is how spike fell in love with Buffy from Buffy the vampire slayer.