Lyric discussion by Zicri 

Cover art for Sleep lyrics by Dandy Warhols, The

It hurts for him to think about the girl because he can't have her. Hence the "but, " in the first line. He can only have her in his dreams, that why he wants to sleep forever. So he can be with her forever and forget about everything else.

@Zicri Good job catching the "but". It is the key to the whole thing.
Right on.