I thought about the first part, Mr. Popper thought that only his Penguins are going to the South Pole, and he wanted them to stay with him. However, he knew what was the best for them so he decided to let them go but when he found out he was going too, he felt happy. Like this, I think it's difficult to send someone you like, however if it's best for them, and you truly love them,  you should let them go.