Vanity, revenge, loneliness, boredom, all apply: lust is one of the least of the reasons for promiscuity.
Mignon McLaughlinA doctor recently described to me "benign positional vertigo": it means you get dizzy in certain positions, but you can get over it without necessarily changing the position. Change "vertigo" to "anxiety," and you've summed up the neurotic's plight.
Mignon McLaughlin