Nightfalls or wet dreams is a very common occurence among men and are perfectly normal. You should understand that seminal fluid is just like any other body fluid like tears urine etc and is being constantly produced by our body.Males achieve their physical maturity at the age of 13 to 14 years while they get a chance of sex much later.During this time one cant keep his semen locked up inside.It has to find a way out.
If you dont get a sexual opportunity ,then you have only two ways by which semen is released.one is masturbation
and other is night fall.Neither of these things is bad or cause any weakness or debility.
There is nothing to be embarrassed about if you suffer from this problem because allmost all men will experience nightfall sometime during their life.
Just relax and if you find night falls problematic ,go ahead and do masturbation occasionally to release the pent up semen.