Listening.
REALLY listening. 
We all want to be heard. REALLY heard. We often prioritise that over listening.  My favourite quote is; "Most people do not listen with the intent to understand; they listen with the intent to reply". How often have you found yourself doing just that? Sitting and listening to what someone is saying, but really thinking about your; emotions or rebuttle - getting geared up to respond. I know I have! That's why with couples that I see, I focus on getting them to listen, REALLY listen. The best exercise I find for this (Couple Therapists if you have good ones too please let me know!) is simple paraphrasing! This exercise is so easy to use and translates to most issues or topics you may be struggling with.
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 All you need to do is; one person be the speaker and the other the listener.
Speaker: Discusses how they are feeling or thinking. Listener:Use your own words to communicate what you understand the Speaker to be saying (paraphrase). Don’t just repeat what the Speaker has said. Listener:: Verify, clarify, or modify the Speaker’s paraphrased response. Speaker: Paraphrases what the Listener has said again.
This is intended to help the listener to know that the Speaker is aware of the Listener's perspective and has heard what they have said. Paraphrasing and restating also allows the Speaker to correct any misunderstanding on the part of the Listener.
Side note; when I was 19 I remember seeing a big poster, and it's always stuck with me. It said; We have two ears, two hands and one mouth-to talk less listen more and do most.