1. Honesty
Yes. Any relationship needs a good amount of honesty. I'm not talking about white lies (why you are five minutes late, talking to your boss about being stuck in a traffic jam when in reality you just didn't leave the house early enough; or the ultimate question of your girlfriend "Do I look fat in this dress?" - "No honey you look beautiful" - to all women here: Have you ever asked yourself why your man always thinks you're beautiful no matter what you wear? Want honest answers, ask your girlfriends, not your man. Don't bring him in that situation. lol).
I'm talking about pure honesty when it comes to the important stuff. Being able to tell the other person what you feel, what you think about a situation and what your ideas and plans in life are - this is the honesty you will need in order to build or maintain a relationship. If you messed up at work, being able to take responsibility for your action really shows how confident you are. And it shows that you are trustworthy that the other person will be able to deal with the situation in an appropriate way.
Side Note: Overreacting and being emotional mostly just happens because multiple little issues from the past days, weeks, months add up, then suddenly explode all at once - you will understand that if the overreacting person suddenly brings up things that bother him/her but have nothing to do with the current argument. Overreacting and being emotional is a sign that there was not enough honesty in the past, otherwise the situation would have been resolved when it occurred.

2. Support
We all need a shoulder to lean on sometimes. We all need support. No matter if its the dinner being made ready to eat after a long day at work, getting picked up after work, or just a simple compliment. Women need support, they need flowers and hugs. Men need support,  a nice massage for neck and shoulders after a long day or even just a compliment like "you look big" after he comes home from the gym. Being supportive shows appreciation of another. It says: "whatever your plan is, I will be there for you and do my best to help you finding success.“ Think about the last time you were supportive towards your partner, your friend, your boss, your employee, your client or any other family member.

3. Acceptance
Take someone or leave someone. But don't try to change him or her. People value acceptance. Someone trying to change us will make us feel like we are not good the way we are. No matter if its your best friend, your brother, your boss. Everyone is different and so are we. If we want to be able to express our true self, we have to accept the others.
This doesn't mean you have to keep living with your friend in a shared apartment while trying to accept the fact that he smokes in the kitchen. If you don't like smoking, you can still change the situation and move out. It also doesn't mean you have to be in a relationship with a jealous person and try to love jealousy even though you hate it. We are all changing constantly. Don't give up on your friends, partners or family members - it might be a phase only, where they do things that you don't understand or like.
However Acceptance means also accepting one's own needs, wishes and desires. If something doesn't align with your own morals or ideas - maybe it's time to move on from that relationship. If you work in a job that totally exploits you and underpays you, quit. If you live in an abusive relationship, break up. You are never stuck in any situation.

4. Conversation
Yes, this one is crucial. You need to be able to talk. To your family members, siblings, your coworkers, your partner, to strangers. Being able to hold a clean conversation exchanging different opinions is so so so important. Unfortunately we never learned "conversation" in school: How to be an active listener, how not to interrupt the other person, how to respond in a good way. Good Conversations give us energy. We learn from good conversations. We understand each other and ourselves better after having a good conversation. A good conversation is like good sex.
An Argument starts in 99% of all cases as a conversation. Person A says something, Person B says the opposite. Simple as that but that's why we have wars on this planet. But how does a conversation turn into an argument? That's when the ego takes over and loads your part of the conversation with emotions. Catch yourself in these moments and become rational. It will help both you and your partner. Arguments simply drain your energy. Rather use that energy and go to the gym. Don't waste it on a stupid argument. Even if you "win" the argument, that doesn't make you richer or a better person.

5. Commitment
Last but not least, commitment is a big thing: If you are making plans with someone, commit to them. If you work for someone, you commit being there in time and putting effort into what you are doing. If you want to travel with a friend, you work out a plan, a route to finding the perfect location where both of you will enjoy the time. If you want to be in a loving relationship with someone, you commit to both adjustments and sacrifices. In almost all situations in live, getting something makes you automatically lose something else. That is probably the biggest commitment in life. Getting a fixed partner limits you in terms of dating other people. Moving to a different country makes it hard to see your family. Breaking up with someone lets you have the freedom to do what you want to do, but you will be alone.
Understanding this will never make you think about decisions the same again. Crazy but true: You will always lose if you win something. And you will always win, even if you lose.