Venerable Ru Jun tackles this question:
Sharing my notes from the discourse:
from the 8:30min mark, the issues we face that affect our mood and that you think are quite a big deal (e.g. something not going your way, some disagreement), tend to be trivial compared to bigger topics like where we will go after our deaths
from the 12min mark, we should always think of the origin of things happening to help in our practice. Even a mundane action like washing our hands, we can contemplate that we are washing a pair of human hands this life. If we didn’t have the teachings of our Teachers, we may be washing our hooves instead (being reborn in the animal realm). Building this habit can remind ourselves of the concept of Karma and can help in curbing our tendency to complain.
from the 14:40min mark, a sign of not believing in Karma is when one uses feelings/emotions to look at things rather than using Dharma to inspect our feelings/emotions. And by not being aware of Karma, we will likely go into the mood of complaining