Around close personal friends: If we're in a clowning mood, it's easy for me to get silly. [Edit:] And I act silly with the wife all the time.
By myself: It's easy for me to be silly by myself, but I forget to do it. I get caught up in my work or my own little dramas or ponder things too hard and too long. So I have to remind myself to get silly and have some laughs at my own expense to release the tension and unwind back to normal. The benefits are obvious enough: It gives me a break from working or brooding and refreshes me, and maybe even gives me some perspective on the things I'm doing by providing some quick distancing. But anyway, as long as I remember to be silly, then the silliness comes easy.
In a crowd of social acquaintances or total strangers: It's always been difficult for me to be silly in social or work situations. I concentrate too hard on making a good impression and avoiding gaffes, with the end result that those environments have always been a strain for me. In recent years I've been working on loosening up and being more natural in difficult social situations, even at the risk of committing gaffes. It's been tough, but it's doable and I find myself enjoying myself much more and interacting much better. I don't worry if I come off as a bit of a chattering clown at first (in fact I cultivate the impression), and meantime the reduction in the stress/aggression/tension level frees up a lot of energy that I can use more productively and creatively in the social interactions themselves. Getting silly in social/work situations makes a big difference in the quality of the interactions. By putting myself at ease, I can cause others to be at their ease and get a better interaction going with them. And that's something that I've only been able to achieve with practice and deliberate intent.