Who exactly is happy? Not the parents, they’re in the hole for more money than the GDP of a mid sized country. Plus, they’re on the hook for the next who knows how many years for who knows how much money (depending on how dependent these children are). Not the guests, either- they’re annoyed to be invited to yet another wedding, 10 just this week, which means no sleep at all since slichos starts at 5 am, plus they’re in the hole for about $100 worth of gas tolls and babysitting, plus they have to give a wedding gift. And they have to pretend that they’re so happy to see all these people they never otherwise see (there’s a reason why they never otherwise see them – they don’t want to). And they have to wait around staring at their empty soup bowls until practically midnight because the photographer is lousy and couldn’t get the “right” shot.
Basically, the only people who are happy are the ones who are paid to be there and sing. And they’re only happy because they are getting money to be there, which goes back to the parent’s unhappiness. So it would really be disingenuous and selfish for them to sing about any current state of happiness.