Why does a nightfall occurs even without having erotic thoughts?

It's normal: By "nightfall" i assume you mean "wet dreams" or nocturnal emissions. These are normal occurrences for men your age. It's your body's way of getting rid of semen, which is produced in fairly copious quantities during the teenage years. If this bothers you, masturbation before going to sleep can help reduce the frequency.

You're a MAN!: You're supposed to enjoy it, Anku. You're a grown man now. You have a wonderful adult body, the power to make a partner happy, to be a loving husband and a proud father. Don't let anybody make you feel guilty or ashamed of the way your body works naturally. If you don't bring yourself to climax while you're awake, by yourself or with a partner, this is going to happen. Be proud of it.