I know my circumstances take a shot at my credibility on my thoughts here, but I would like to offer them anyway, for whatever they are worth. When I was housebound for the longest period in my life, i thought long and hard about this, questioning why we should have friends at all! and it just came down to the fact that we are wired to be social beings, there's no getting around that fact, we observably do worse in isolation. It's in us to want companionship, to share our thoughts, feelings and experiences with others. This then turned my attention to how we make friends and how to maintain a friendship.
There are numerous ways to make friends. For one, you can draw on the contacts you already know to branch our further and meet new people. This may be the easiest route for some, as theirs a point of familiarity in the people you already know, which will also make the other person more open to you. Though, there are times, like mine at present, where you have to start from scratch. Work and school are two big opportunities to make friends. Or you can take up a new hobby and put yourself in new situations where you will naturally meet new people. The possibilities are boundless, how about the internet! (Do be careful with that one).
Next is the are I struggled most with. As a severe SAD sufferer, it was such a struggle to get through each day that I became very centred on myself, how I felt and how others acted around me. I became side-tracked by my issues as I became more withdrawn. The biggest thing you can do, in my opinion, is rework how you treat others. Remember all of those reasons for companionship I listed above? Well the people you meet are looking for the same thing. I believe that many times we lose friends because they end up doing all of the work, and eventually they take that as a hint to move on. so take an interest in others, invite them out, be pro-active. Show an interest in their lives, and show them that you value them and want them to be a part of yours! Instead of focusing on what you want to receive from others, give to them everything that you would want from them. Do this and I doubt you will lose any true friends.
I hope this post wasn't too preachy, I sincerely wish the best of luck in your endeavour for companionship SM
Dosvedanya