I don't think people realize how big of a gentleman Spassky was. He deeply respected Fischer, even though Fischer never really returned that respect. Spassky could have retained the title before their 1972 match even begun by simply walking away from it because of all the complaining Fischer did. Not only that but he had ANOTHER chance when the 3rd game started in the small room away from the cameras, where Fischer was still complaining. Deep down Spassky knew he was going to lose the match, and that was all the better to him I think because he didn't really seem too concerned with retaining the title. Karpov helped him train before the match and said most of the time he would just play tennis and go swimming.
Then in their 1992 rematch Spassky had a chance to beat Fischer in game 6 which would have been his third win in a row because he won games 4 and 5 as well. Spassky decided to draw game 6 "for the sake of Fischer's return" so Fischer wouldn't get too frustrated and abandon the match, because after losing games 4 and 5 he started showing the signs of old Fischer and was beginning to make demands again. After the draw he then lost 3 games in a row to Fischer. Kasparov felt this was a sign of Spassky's "magnanimity".
Spassky really liked Fischer. He was less concerned about chess and more concerned with helping someone who he felt was his friend, and if he could make a little money from the matches, all the better.
Lasker and Kasparov held the title for many years.
Kasparov did it it against more titled players in an era where technique is much more refined.