If you dont use your muscles then they think they do not need to be as large and so they start to breakdown and shrink. This makes them weaker. With age and in obesity the blood contains immune cells at a higher level than young healthy people and these can also breakdown your muscle!
Muscles get weak for lots of reasons – as we get older (from about 30 years old onwards) muscles get weaker. This is because of lots of different changes in your body – your nerves (which control how your muscles work) become worse at telling your muscles to work strongly, and you have less of the chemicals which tell your body to grow. You also eat less (the body needs lots of protein from food to make muscle), and the body isn’t as good at turning the food into muscles.
Like Stuart said, if you don’t use your muscles (if you don’t exercise) then they’ll get weaker. This is because it takes lots of effort and energy for the body to keep strong muscles, so if you’re not using them the body can’t be bothered making all that effort!