Simple Suggestions: 1: Turn off the TV and the computer. 2: Invite some friends over for boardgames/storytelling/joke sharing. 3: Learn to sing/play and instrument & join a group for practice. 4: Learn to dance and throw a dance party. 5: Join a group that seeks to improve a situation you are concerned about. Basically entertain yourselves and act the way our great-grandparents did - with each other rather than with expensive technology. Meetup.com is a good tool for meeting real people with similar interests.

To your first two questions: fear and improper communication.

Fear is a powerful drive. Fear of judgment, of ridicule and rejection. Or sometimes it's fear of being hurt, especially if you've been hurt before. All the can cloud our judgement and rein us in from going after what we want.

Improper communication is when something seems
to be obvious or clear to one person, when it's anything but to their partner. When we say "It's fine.", when it isn't and refuse to talk about. When we want or need something but don't say it and get frustrated that they don't give us what we're looking for. All of this serves to drive people appart.

For your last question: trust, proper communication and faith.

Trust is so fundamental that without it you don't have a relationship at all. You just have a series if interactions that will most likely be bad and harmful. You can trust them not judge or hurt you. Without trust you have no real basis by which to communicate.

Proper communication is what is born of trust. Judgement free exchange of ideas, hopes and fears. The ability to say what you want in a way your partner can clearly understand and they do the same with you.

Faith is one I imagine most people here will bristle at, but hear me out. I'm not talking a higher power. I'm talking faith in yourself and your partner. You can never truly know another person. There will always be some part of them that is theirs alone. Which means at a certain point you have to have faith that your judgement was sound, that your trust wasn't misplaced, and that you expressed yourself clearly. You have to surrender some modicum of control, pass the ball to them and see what happens.

All that are alone are not lonely. Just wanted to make that distinction first.
Here is my theory and it is halfbaked, even though I am not at this time.
Lonely is the state where I find myself longing for what is past. That deep connection I shared with someone. Dating can make it worse because we wonder why we can't/don't have it with the new person. It takes time and often the relationship ends before you find it for one reason or another. I think that makes people less likely to expend the effort, or at least I know that is the case for me. It seems like a loop and it is exhausting. Emotions themselves are draining at times and it is hard to always be on. I think socially or culturally we wait too long before trusting people with who we are interested in. So I just trust people aren't out to screw me over and I know that is never my intent.

Dating is the pretty crazy ritual when you break it down and is fundamentally flawed imo. I listened to all my girlfriends talk about how shitty it was all through my teens 20s and 30s. I never really dated in a traditional sense save for a bit maybe in high school. I tend to avoid it for the most part. To me relationships are built on trust so I just try and meet someone have fun and try it again. If you can keep repeating that formula things work out how they are supposed to. Maybe that no longer works but I think that is the only thing that feels right now. Selecting from a list just isn't for me right now so I will just try to be a fun person to hang with.

I am alone and at times lonely like I said above so I need to be better about dropping the past. I probably won't always be a fun person until I can be better at that.

Everone is holding out for a 10...