Yep, time...but sometimes time takes a long time (see Giok's post above). One too many times for a heart to be broken is the root cause of cynicism. There's a famous quote/line from a song/ - or something to that effect that says essentially the same thing far more eloquently than I just did. (maybe it was Jim White's "10 Miles To Go On A 9 Mile Road") see lyric about 2/3 down the page   

Basic tenet of Buddhism (?) is, the root of all suffering is desire. Eliminate desire, and you eliminate suffering. What a simple cure for most of the ills, individual and global, that plague the planet.