One of the things that can prove most challenging when you're close to someone, whether it is as a life partner, a mate or a work colleague, is accepting that sometimes it simply just isn't about you.
It can be tempting, particularly if you're a 'fixer' like me, to want to offer a solution to someone's bad day, or to even feel guilty, and assign blame to yourself for their bad mood. We all lead complex lives, with such a tapestry of influencers and influences on our feelings. Hormones, weather, work, the commute, a book, news from friends, tiredness, illness etc etc, all can hold a large sway over all of our emotions.
So when we're shrugged off with "I've had a bad day", allow yourself to just accept it as true; they've just had a bad day.
An off-mood in a loved one doesn't have to feel like a code for a bump in your relationship road, nor does it need to be dwelled upon. If it's just a bad day, this too shall pass.