Going it alone is not the way to a long and fulfilled life. We live longer when we have a good social network, and emotional support from friends and family is not only an important quality of life component, but also better for your health and mental wellbeing. With life expectancy on the up and up our prospects for a long life are further improving if we can develop and maintain a solid social network.
But building one, and then keeping it alive, takes work. It's pretty easy to make friends anywhere you go on a regular basis, where you run into the same people over and over. Any committee in your community, that does something that tickles your fancy, will do, any organization with regular meetings, any cultural or sports activity around town, all sorts of school activities (if you are a parent of younger children), church or temple of course, gyms and exercise classes, community events - so many opportunities. For retired people volunteering is really meaningful because it feels good to help and be useful.
Maintaining your network is key to remaining engaged and always having a good social network to draw on. In order to get invited you need to invite, in order to have a conversation you need to engage, in order to remain in your friends' minds you need to remind them. And kindness follows you. Call, send birthday wishes, ask your friends to do something with you, drop by their house for tea, offer your help. It's reciprocal. If you don't reach out they may forget you or think you are not interested.