I have always been a bit of a worrier over my dc, of course who doesn't. I want them to be happy (again of course).
DS had a rubbish year in parts last school year due to low level but prolonged bullying which I guess has made me a bit more worrisome.
Anyway yesterday walking home from school with ds I said "you seem like you have a great day, you're really happy"
He agreed with me, and I said "that's nice, because yesterday you seemed a bit gloomy" (me thinking oh what's happened,why is he sad, has he fallen out with his mates etc etc 
He replied "no one can be happy ALL the time, that would just be sooooo weird. You need to have not good days so the good days are good"
Put it all in perspective for me. If he can deal with a duff day, but realise that's just it, one duff day or one day of feeling a bit gloomy, then he really isn't going far wrong.