As for Angel...it's the thing that happens in a few people. Sometimes, you can meet a person in your life and something inside screams "he/she is the one for you...you can't imagine life without that person"...this is how I feel whenever I saw them on screen! I won't analyse anything about them, I just believe that when i see them together, I sense something so strong that it is impossible to neglect.
(I know that this is going to sound condescending when I say it, but please, PLEASE, believe me when I say that it isn't. It's just the opinion , knowledge, and experience of someone who has been alive for three times as long as you have.)
I've seen people who were "made for each other," who quite literally fell in love the moment that they first met, who never even considered sharing their lives with anyone else. It's rare, and it's wonderful. It's the stuff of legends and dreams. Unfortunately, like legends, it's at least partially mythical, and like dreams, it's something that the dreamer eventually wakes up from.
First love, and that's what was portrayed in the Bangel relationship, is rarely, if ever, last love. It MAY be, but doesn't have to be, the greatest love that we ever experience. It almost certainly was Buffy's first real love, and it more than likely, from what we have been shown in flashbacks of Liam's human life, the first time that he has ever loved anyone or anything, at least anyone who openly returned that love. (Maybe he actually loved his little sister. We know that she loved him, but any reciprocal love remains pure conjecture) Neither Buffy nor Angel had anything to compare their feelings to. It was love, and it was real, and it was something that neither one had ever really felt before. That's enough to convince either of them that the other is THE ONE.
Probably the greatest single thing about first love is that without it, there can never be a second love, or a third. First love is the very foundation on which we build every subsequent relationship that we will ever experience. It's the standard we will always measure those later loves by. But, as the poet said:
What though the radiance which was once so bright
Be now for ever taken from my sight,
Though nothing can bring back the hour
Of splendour in the grass, of glory in the flower;
We will grieve not, rather find
Strength in what remains behind...
Buffy and Angel were brought together at a time in both of their lives when they both desperately needed something that neither had ever had before. What they had has been been the key to success of Harlequin Romance Novels for decades. Unfortunately, real life goes on after the last page of the romance novel has been read. Angel left Sunnydale at the end of Season 3, and it almost killed Buffy. But she got over it in time to form a relationship with Riley, a relationship that was, admittedly not one of the legendary proportions as what she had shared with Angel, but, by her own admission, he was "someone who she could trust," even if she meant only that he could be trusted not to wake up the next morning a monster intent on destroying the world.
The fact that what Buffy and Angel felt for each other in Sunnydale has run its course and is in the past in no way precludes the possibility that they might meet again as a mature man and woman and fall in love all over again. It simply means that what an adolescent girl once felt for a handsome, mysterious, tortured man, and what a lonely man saw in a young girl who was bravely facing the prospect of an early death after literally protecting the world from all of the forces of hell is now part of each one's past.
Adult, mature Buffy may one day be reunited someday with an Angel who has finally reconciled himself to the horrors and sins of his past life, and that woman and that man may finally be ready and to live their lives together, But that's sometime in the future, exactly what Buffy told Angel in that cemetery in Sunnydale the last time that they met. Angel might very well be the one who is the recipient of Buffy's cookies.
But in the meantime, Buffy loves Spike the best.