I realize that I use the terms happy and sad to measure the level of comfort I'm experiencing at the present moment in life. Happy = most comfort; sad = least comfort.
I don't think any class in school adequately prepares a person for the reality that life is not going to be 100% comfortable all of the time. I think one of the biggest detriments to humanity is being sold on the delusion that 100% comfort, 100% of the time is even attainable. We're so obsessed with comfort in the West that we think that is the goal of life. My mind has succumbed as victim to this mentality.
The goal of life is not to be 100% comfortable. In fact, I'm beginning to agree with the notion that that's quite boring. Instead of happiness being a measure of comfort, happiness (peace, being centered, etc) is something that one should choose to be even when situations in life causes one to be less than 100% comfortable.
Just as happiness is a choice, sadness and/or suffering is a choice also. With any situation in life, the situation does not cause you to suffer, instead you choose to suffer through the situation.