I think it easier for animals to be happier than us just for the mere fact that their intellects are factually lower which means their emotional intelligence is also lower... of course this doesn't mean they can't feel sad or other emotions, they just don't feel them as deep or punish themselves as much as we do over small little things...

I sometimes still grieve about decisions i made or didn't made 10 years ago, i doubt your average animal does that...