San Diego has the best climate in the USA, and amongst the best in the world IMHO.
Summer is glorious. It's hot, but a comfortable dry heat unlike (for example) Florida which can be hot, humid and sticky.
Winters are generally very mild. Not sunbathing weather, but warm sunny days.
When it rains, it rains very heavily - but it doesn't rain very often.
It's also one of the most beatiful areas in America.