It's all your set and setting, man.

I find that if I'm in a bad mood to start, or even slightly agitated, it's not a good time for me to smoke weed. It always seemed (to me anyway) that marijuana is a mood magnifier. Basically it just exaggerates any mood you're in, to a higher degree.

It probably doesn't happen to you now because you're more comfortable smoking it, and you've learned to just relax and always have a good experience. Keep on tokin!