It's hard to answer without knowing what you consider "rational". You're asking about an emotion, and a negative one at that. When is being upset ever "rational

It's hard to answer without knowing what you consider "rational". You're asking about an emotion, and a negative one at that. When is being upset ever "rational" in your mind in the first place? It's pretty nonproductive as far as the external world is concerned, and it sucks away your time and energy. The way you've posed it I don't see when it's ever rational, unless you're the type of person who thinks "I'm going to cry anyway, so I'll set myself a reminder to cry when I'm free at 5pm today because otherwise it'll interrupt my work tomorrow".