Well, I live in Asia, so examples are mostly Asian.
From the stories I've heard are:
1) You walk through the graveyard or funeral saw a picture of the pretty lady and said I wouldn't mind marrying that chick/guy, or mann it would be great to marry that person. Then the spirit hears that they follow you come and come to your dreams every night supposely take away Energy. As day goes by, you grow weaker and weaker. This is a typical Asian Chinese story.
2) You marry a girl that is already dead and visit you at certain times. Never knew why that never happens to widows and single married men.
3) I've read of stories on the net of how one person forms a relationship with sucubis/ incubis, and they would visit when one calls or on a planned schedule.
But from a Chinese saying, spirits/ghost belongs to Ying and humans Yang, which is life force, the more often you stay with a spirit the longer weaker your Yang becomes.
i suppose you mean someone dating a spirit. yes, it can happen. basically, since most spirits were once people, they hang around other people, sometimes they make living friends. as with any friendship, people sometimes grow close, and sometimes someone 'like likes' [love that term] the other person. you ask the person out, they say yes or no, stuff like that. i'm not saying you should, i'm not saying you shouldn't, but people should think about it before dating a spirit, first of all, they're dead, so you can't really do those normal relationship things. second, try breaking up with them, it can get a bit more annoying and dangerous. myself, i've never dated a spirit, i am really close with one though, and i have met people who are or have dated spirits. so, yes, you can date spirits, you just ask, but i wouldn't recommend it because they're spirits. i'd give the same advise for someone wanting to date a demon, you can but why?