Emotions are totally a responsive phenomenon and are the result of an environment and circumstances. You cannot choose your emotions. That being said, your environment will naturally include your thoughts and attitudes - and these do play a big role. Indirectly, you can influence your emotions. If you're mad, you can think calming thoughts; if you're sad, you can think of what you do have. You can't directly say "stop being mad" or curb that sadness at the neurological level (well, except with narcotics), but you can alter your voluntary thoughts to affect your involuntary emotions. The ability of people to do this, like with pretty much everything, varies significantly, though.