I find it difficult to close myself off

I notice mainly that I find it difficult to close myself off. My partner is often very emotional. And that effects me. For example when I had a fantastic day, I come home to a partner that is angry and then my whole day is also screwed up. I would like to let that slide off me. But I can't.

Why can I not just let his emotions with him, without bothering about it? PS it is not about major things or anything, but just that he is angry because internet is not working, and that ruins my day. You understand?