He found Sophie, eventually, in the photocopier room; ... He looked at the back of her head and thought, Only you, you and no other; and then he asked himself, Why, for pity's sake? What's so special about her, as against, say, Vicky or Demelza Horrocks or even, within certain firmly defined parameters, Mr. Tanner's mum? Paul thought about that for two, maybe two and a half seconds, and realised that he didn't know, that there wasn't an answer; just as there's never any answer to those simple but incredibly difficult questions you ask when you're four years old and the grown-up just looks at you all cross and embarrassed and says, "Just because, that's why." (And, of course, that's the only real, true answer to that kind of question; there are long-winded ways of saying the same thing, involving lengthy digressions and background materials and abstrusive and nebulous concepts, but they're just another way of saying that same old thing).
Why? Because. And that's how you know it's true love. If you love someone for or because of something, it doesn't count; it's admiration, appreciation or the recognition of some resources she's got that you can exploit to your own advantage. Unless it's just Because, it can't be unconditional, instinctive, involuntary. Doesn't count. No good.