I know a person who keeps saying that he likes to be sad and just live his life without any enthusiasm or motivation. He prefers to be alone and he doesn't want to engage in activities that are fun-filled. Is there a word which can capture this description?
I don't think there is a single word for this. Melancholic is close, but doesn't really imply a desire to remain so. Calling him a loner is also close but doesn't fully denote a lack of enthusiasm or motivation for anything he keeps to himself.
However, it does sound a lot like self-defeating personality disorder:
The person may often avoid or undermine pleasurable experiences [...]
[and] rejects opportunities for pleasure, or is reluctant to acknowledge enjoying themself
There's more to it than that, but since this is not an appropriate place to practise amateur psychology I will leave it there.
Unless this person is suffering from what was mentioned already I think you'll need more than one word to describe him.
Introvert would be an obvious first choice. Introverts can be hard to get close to and they can seem very shut off from other people. Depressed is certainly a possibility.
"Emo" is an option, but this is such a negatively loaded word that I wouldn't recommend using it.
One (slang) option is downer. This is more commonly used either for depressive drugs or experiences, but it can be applied to people as well:
- A depressant or sedative drug, such as a barbiturate or tranquilizer.
- One that depresses, such as an experience or person. - thefreedictionary.com
You could call him an Eeyore in reference to the donkey character in A.A. Milne's Winnie the Pooh books (and more recently in the Disney film derived from them). This is a fairly recent popular usage but would be recognised by most people. One positive aspect of using it is that it would be pointing rather gentle fun at the target.