The weather is horrible, there's loads to do, it's not the same as it was when you were younger because you have to do much more work to make things enjoyable..
Also - everyone seems to be happy, you feel as though you should be happy, and if your MH isn't great why should your illness disappear just because it's Christmas?
You probably struggle as well with the fact that you have very happy memories of Christmas, and find it hard to cope that it isn't as magical or as exciting. Or if you remember it being the happiest time of year, you may struggle that you're not happy as you used to be. This is something that everyone has to go through as they get older, but MH problems exacerbate it somewhat.
Most people manage to find things that they enjoy about it. For example, I now know that I won't get thousands of presents that I spend the whole year being excited about (though I do get very spoiled) and I know I will usually be able to get to sleep on Christmas Eve, when in the past it would have taken me hours and I'd have been staring at my alarm clock willing it to go faster so I could get up.
Christmas as a child is the best feeling most of us will ever experience, and it's hard to adapt when that goes. But once you do adapt, you learn that are lots of good things still. For example I love seeing all of my family, watching rubbish on TV, eating various turkey dishes for a week, seeing other people enjoy gifts that I have got for them.
Your MH is no worse at this time of year, but the emotion of it may make it feel worse.. If that makes sense.