A list of ways in which we are able to inflict sadness
Spinoza has the answers to any and all questions you might have about happiness, sadness, and the point of this life of ours. In short, he sets out to prove that we are simply here to persist on existing. Following that, he writes happiness occurs when we improve our capacity to persist on existing, and inversly, we are sad when we diminish it. He continues by defining love as happiness (improving our capacities) with someone as an associated cause, and inversly again, by defining hate as sadness or suffering with someone as an associated cause.
Now, I would like to list ways in which we are able to inflict sadness, either on someone in our surroundings or ourselves. To inflict pain on someone, according to Spinoza, is to cripple their capacity to persist on existing. Violence, harrassment, theft, are obvious instinctual examples, but there are subtler ways. You learn about these only when you've experienced them, either as the victim or the perpetrator.
Here's an ongoing list: hiding errors you've made, overlapping territories, not asking for help, asking something of someone without explanation as to why it's needed, never questioning facts brought forward, being unaware of love or hate, being unaware of ton of voice, overtly hating what another loves, unreasonable adversarial or accusing stances...