Convincing others that anything is fun to do is quite tricky.
I have a kid that does not want to have anything to do with painting, and I tried to convince him he can do it, but he's stubborn on his refusal. So, trying to convince people that painting is fun does not always work, but it's worth a try.
That said, I can think of two analogies that may come in handy.
1. Appetite and food enjoyment.
Even if we are not hungry, seeing someone eating with true pleasure and enjoyment can make us crave that food. I remember as a child seeing a cartoon of Vicky the Viking where the hungry Vikings devoured huge roasted thighs, holding them with their hands and taking big bites out of them. Those scenes made a life-long impression on me. I still crave roasted thighs now, after many years.
2. Bob Ross.
He made millions of regular people think that painting was easy and fun. How did he do it? He showed how much fun he was having creating and that making mistakes is not a big deal, you can fix mistakes or turn them into opportunities for unplanned beauty.
The two examples have one thing in common: both the Vikings and Bob Ross were truly enjoying what they were doing, and that showed through.
In short, I think the best way to convince others that painting is fun is to do it and talk about it with passion. Show them how much you enjoy painting, talk about how it makes your life happier.
Talk about painting with a sparkle in your eyes and a grin on your face, and they will want to have that joy in their life.