Jessica Simpson has learned something from her past relationships with the likes of John Mayer, Nick Lachey, Tony Romo and (briefly) Billy Corgan. "You can't love someone else to change them," the now-unattached Simpson Tweeted Sunday. "Happiness comes from accepting who they are," she continued. "It is our decision to REALLY know who we love."The 29 year-old star's most talked-about former love is arguably Mayer,32!