This is a broad topic that I'll call, "At the Core, it's the Heart." It will answer the question, "What are characteristics of the heart?" Can you name some?
1st, your heart should be carefully guarded:
Watch over your heart with all diligence,
For from it flow the springs of life. (Pr 4:23)
We have to be careful with our hearts, because having damage to them damages one's entire life.
2nd, your heart determines your character:
For as he thinks within himself, so he is.
He says to you, “Eat and drink!”
But his heart is not with you. (Pr 23:7)
Kind words don't matter if not backed up by action. In being careful with our hearts, we also have to observe the actions of others that proclaim to have desire for us. Especially in romance, people can get stuck in abusive cycles by listening to the words of the abuser and not the actions.
3rd, it is the source of defilement when not cleaned:
"But the things that proceed out of the mouth come from the heart, and those defile the man."(Mt 15:18)
This is basically the same as the next one, but the heart is our character. If it's harboring some oily, dark problems, expect them to be manifested in our lives. But if it's pure and upright, expect to see it shining no matter what.
4th, it controls your speech:
"The good man out of the good treasure of his heart brings forth what is good; and the evil man out of the evil treasure brings forth what is evil; for his mouth speaks from that which fills his heart." (Lu 6:45)
Ever had someone hurt you with words on purpose? Evil words are just that: evil. They don't spring up out of nowhere, but rather flow from the heart. It's fine to be honest, but it's not fine to seek to do damage to someone else. Ever. Satan's the enemy here.
5th, it is the source of belief:
"for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation." (Rom 10:10)
This one is important. We can go through every nice action in the world, but if our heart does not have conviction, there is no belief, and it's all for naught. It's the same in ANY relationship. If your heart isn't in it, it's going to be hollow; a facade filled with nothing. No happy memories, nothing.
6th, it can be a deceitful thing:
“The heart is more deceitful than all else
And is desperately sick;
Who can understand it?" (Jer 17:9)
Man, this one always hits home. For me, it's in relationships. I used to be drawn to people that I knew weren't good, but I said, "Hey, that's love for you! Can't help what my heart loves." How wrong I was. The heart can be incredibly deceitful, and we need to rely instead on the mind of Christ. (1 Cor 2:16)
7th, it can be stricken by failing to obtain something desired:
"Hope deferred makes the heart sick,
But desire fulfilled is a tree of life." (Pro 13:12)
Here's a good quote to sum this up: "Delay in the accomplishment of some much-desired goal occasions sinking of the spirits and despondence; but, when the object of longing is obtained, it is a tree of life."
This is the same in our love relationships with Christ and with our marriage partners. If we delay being immersed and receiving the Holy Spirit, we often encounter needless heartsickness, because we leave a hole in our lives. The same thing can be said of a man not going for his suitable helpmate.
8th, a broken heart is a true form of illness that affects even the strongest person:
So the king said to me, “Why is your face sad though you are not sick? This is nothing but sadness of heart. Then I was very much afraid.” (Neh 2:2)
A sad heart is the hardest wound to heal.
9th, hearts can be broken, but they can also be healed and strengthened:
Reproach has broken my heart and I am so sick.
And I looked for sympathy, but there was none,
And for comforters, but I found none. (Psa 69:20)
"For it is good for the heart to be strengthened by grace." (Heb 13:9)
Luckily, God can heal any damage to us, including to our hearts, and he can strengthen us beyond our comprehension! :)