Is it normal i wonder why many disabled people seem very happy?

They're often not. People in the worst situations often seem the happiest because emotions are contagious and they don't want anyone to feel they way they do.
When people are miserable for so long they eventually learn to forget because laughter becomes necessary for survival.

It seemed pretty obvious to me. I could be wrong of course.

You probably take a lot of things for granted. A lot of people do, with or without realizing it. So many of the people you're talking about probably did as well...until they almost died. They got another chance. They are grateful that they got to see their friends and family again. They were strong enough to get through whatever it is that they got through and now live each day to its fullest potential. They are happy just to be alive . Even for the ones who went blind or deaf or whatever, they still have their life to be thankful for.