DS is 6, will be 7 in July. He's a lovely boy in a lot of ways...very clever and funny, etc.
However he always, always focuses on the negative. For instance today he had a decent day at school (did a project he was excited about), got a haircut after school that he thinks makes him look really cool, got candy at the hair dresser's, got to play with a friend of his while I chatted with her mum at her mums's shop, got to play outside for a long time this afternoon with other friends, got to watch his favorite show...long story short, he's had a good day.
This evening he's been mopey and grouchy and whiny. He keeps saying he's had a rough day. All I can get out of him as to why, is that his side hurt when he was running at recess. 
Also he thinks his project wasn't as good at two of his classmates'.
I just don't know. He could have the most amazingly fun day ever, then stub his toe and declare the entire day ruined.
It's hard for me, I'm very much a "look on the sunny side" sort of person and also was raised to be very resilient. I worry for his future happiness with this constant negative attitude. I have an older brother who is really negative and argumentative and he is a really unhappy person. I don't want that for DS. :(
DS's first reaction to anything is "I don't want to" even if it's something hr enjoys. A friend invited him over after soccer last weekend and DS really didn't want to go despite always having fun there. I ended up getting him to go and he had a great time, but I know if it's sprung on him again he will automatically say he doesn't want to. He is cautious and tends to brood. DH and I aren't like that but I've just always thought that DS just is like that, and it's fine. With the negativity, do I need to just accept that he has a pessimistic attitude and go with it, or is there any way to get him to be a bitmore positive? I've tried to just "jolly him along", and occasionally when it's been really bad (constant complaining) I've had him write down a list of the things he is happy about or grateful for.
Part of it is, I don't want him perceived as a brat. He really isn't! But sometimes he seems ungrateful or grumpy.