I have developed this theory about sentimental relationships which seems to function rather well also in building friendships:

There are various things which human beings may share. Some have a deep influence in building relationships. Apart from language, profession and hobbies the more we touch the sphere of ideals, metaphysics and so on, the easier it is to socialize on that basis. In that case , the Context is shaped by shared beliefs and ideals.

So, if you are alone ,rather than a gym , join a Church, an animal shelter, an animal protection group, a human right group or also a political Party (especially at a local level). Animal Welfare Groups are good because it seems to me that a pet-loving man is seen as noble and reassuring by women (and also the reverse phenomenon is possible). idem about helping the homeless or the orphan. It will also help you find new meaning in your life, anyway.

It is also true that the more extremist is a group the deeper are the ties among the participants . Indeed, that is true I think, especially in groups who are deeply anti-social because those ideal pit the members against society as a common enemy and there are shared secrets (pls observe that many mainstream Religions , like Christianity, started like that).But I am not telling you to join the Islamic State , the Moonies or the Westboro Baptist Church! Ok?

So, let us forget nutty extremists : Churches, para-Churches, Mosques , Buddhist Temples, NGOs helping homeless men, animals and the environment are good to find new friendships. Especially if they are small, have a clear identity and is organized with local chapters.

I am opened to discussion, of course.