Loss of interest, energy...save it. Read "The Manipulated Man" by Esther Vilar printed by Pinter and Martin for a real education. Flunky woman libers and other "entitlement type" women hate the book so "deeply", you know it's great. It's extremely candid by a very, very candid woman. Otherwise, the absolute safety of a vasectomy and the decision to be married tends to rule the game.
Unless I have kids to raise and I'm providing the ideal nuclear home environment, typically, I'm way, way happiest as a bachelor. Girlfriends? Always be doing something special together. Bust your nuts along with her bra straps laughing out loud together anytime you get the chance. Share many unique, special moments together - mostly funny.