Communication
Communication is not a small thing in a relationship, I'll give you that. It has many forms and is hard not to touch upon.
1. Silence is Not Always Golden.
Key and Peele
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Choosing not to talk about what's bothering you. I know people deal with things that upset them in their own way. If you don't feel comfortable saying what you really feel or feel that saying something will just make things worse, you will probably have this weigh you down in the long run. Relationships take sacrifice and sometimes that sacrifice is the time it takes to have a conversation about the things that are bothering you. That person is very likely not a mind-reader.
2. You Have 2 Ears...
Key and Peele
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How you choose to hear each other. This one is more for people who have been in a relationship for a while. When you feel like you know everything about the other person, you sometimes forget they are another person. It's important to remember that their problems aren't always the same as your own and neither are their priorities. If someone can't think beyond their own issues or try to find common ground from time to time, they might cause strain in their relationship.
3. More like 2.5
Key and Peele
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Additionally, if you know of the winner-loser dynamic, you know that there's usually that one person in the relationship who always "wins" an argument. If a person is choosing not to truly hear their significant other because they want to get their way, it will likely leave the other person feeling bitter.
4. Call it What You Want...
Key and Peele
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Keeping secrets is an obvious no-no. We all know the golden rule from kindergarten: "treat others the way you want to be treated." If your game plan in your relationship is to be honest about everything except that one time you went one place but actually went somewhere else, then expect to have them do the same for you.
5. Talk Trounces Text.
Key and Peele
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Body language is kind of a big deal. So is tone. Both of those things are pretty impossible to completely replicate via text. It isn't always an issue, but when it becomes one, it causes unnecessary stress
Communication affects relationships of all sorts, not just romantic ones. Be sure to be mindful of the emotions and priorities of others as you talk to them, especially if the subject is sensitive.