So I've noticed one of my friends, we'll call him Mark, hasn't been himself, lately. Mark is usually very down to earth and very optimistic guy, and lately he has been really depressed. When I first asked him what was wrong, Mark brushed it off saying he didn't want to talk about it and that he would be fine. So I respected his wishes and dropped the subject, however his behavior never changed over time. I asked him again, quite sometime after the first exchange, and he said that he has been feeling lonely lately. He hasn't been in a relationship or even had sex in over 5-6 years and that he's feeling stuck and not attractive. I asked Mark if he was ever interested anyone and he said that whoever he has found attractive either likes one of his friends or is just not interested in him. Because this has been a constant cycle for him, Mark said he 'gave up' on looking, but every now and then gets caught in his own thoughts. Mark also expressed how he is starting to become anxious when trying to flirt with people and shuts down.

I'm not exactly sure what to say or do. I just listened, I was at a loss for words. I've never seen him so down before nor would I have guessed he felt so alone. Do you guys have any advice?

TLDR: My friend hasn't been in a relationship for a while and is starting to get depressed, and I don't know what to do.