I've noticed this show is really careful about the types of lessons it tries to teach.
Nothing is simple.
People often can't be talked out of things (the king under the effect of the potion, varian), you can't make everyone like you and that's fine, emotional conflicts are treated in a human way, they still affect the characters deep down, and are treated seriously.
The queen didn't just forgive Eugene for that theft, she admitted the experience was traumatic for her but she still did her best to see that he's not the same person he once was.
People have different needs and that's fine, and they should be respected, and no one's wrong for it.
Eugene would've been fine to settle for his best possible life, but Rapunzel needed the adventure and needed to see the world for herself.
After they get over their slight selfishness in seeing the problem, they can see where the other is coming from.
And the whole season one is FULL of the theme of not being ready.
It is said to Rapunzel, she's not ready to know the truth, to go outside, it is said to Cassandra, she's not ready to take on the responsibilities of a royal guard, it is said to Varian, he's not ready to know the truth, he's not grown enough.
So when that final song comes, it's not out of nowhere, it's the culmination of this big theme, and they all get to sing it.
I love this show so much, I could go on but I'll stop here.