Because they're afraid, and they want to scream and cry like tantrum children.... I'm not a parent, so should we just let them cry it out at this point, or yell at them, or give them a big hug? I think my own mom used to mostly just let me wear myself out when I had a tantrum, but I don't know what the current parenting trend is.
Leave 'em on the floor and ignore them.
But seriously, I guess the emotion of the moment and the reality that something they've waited for years for didn't play out anything like their utopian visions temporarily clouds their judgement, releases all kinds of pent up (but overreactionary) fears, and keeps them from immediately seeing all the good aspects, just because it is different than expected. Kinda similar to why many brides cry on their wedding morning, only to afterwards enjoy a long and happy marriage.
If you're at home, let 'em scream it out. If you're in the restroom at Olive Garden, strap them back into the car seat and let them scream it out.
FWIW, I have now officially done that with both boys...
you should try strapping a 16-year-old in a car seat...
Well, if mine threw a tantrum at home, they got one whack on the fanny to break the pattern, then got sent to their rooms where they could yell all they wanted, but I didn't have to listen to them. They knew better than to do it in public. They figured the same thing would happen, but they never pushed it. If they ever even got close to doing it, they got taken to the car and sent to their rooms when we got home.