In the case of young adolescents going through puberty, having sexual and\or romantic dreams about the person who have a crush on is a common thing. Often, dreams are caused by thinking about the person, or thinking sexual thoughts about that person, before falling asleep.
It could also mean that whatever happens in the dream symbolizes specific feelings. For example, if a young girl had a dream that her boyfriend would not kiss her, it could express that she was feeling frustrated and romantically deprived because he was not making sufficient romantic moves to satisfy or please her.
Does it have an effect on your health? The answer is most often no. You may suffer some loss of sleep, but unless the dreams are particularly vivid, startling, and short, it probably will not effect your health unless you are lusting after the person constantly.