You had a bad day.
Things didn't go as you expected.
You were annoyed that the dog kept you from sleeping through the night.
You hit every light and were late to an appointment.
The line at the gas station was long, and you were late coming home.
You are tired, emotionally spent, and ready to just zone.
[enter your partner]
You feel the frustration in the air that dinner isn't ready and think "he's so impatient", "she can be so irrational"...
Do you find it interesting that perhaps your reaction has a justifiable rationale ("Yeah, that may sound insensitive, but I had a really bad day"), and your reaction to your partner is more related to her character than the experiences of her day? Your irritability is justifiable and your partner's irritability is "because my partner is selfish".
Taking a deliberate objective perspective puts a "benefit of the doubt" into the picture. You no longer experience your partner's reaction as a character flaw and take a moment to give him the same justifiable rationale as you have given yourself.
Simply put: When all else fails, just ask your partner, "how was your day?" before responding to what felt unjustified or selfish.