They tend to make excuses for you when you don't explain yourself. They don't believe in being "too busy" for you. They make time even when you don't. They accept apologies you will never give. They see the best you could be rather than what you are now. At your worst, they do their best to lift you up, to help you, even if it means making sacrifices and putting their priorities aside. They accept the love they think they've earned. And sometimes you may question why they are so caring, why they are so willing to give so much of themselves.
However, there is a day when a good heart does give up on someone who is undeserving. There is a day when it stops beating for you. That is when it's been neglected for so long, when it's been mistreated for way too long. A heart does not turn cold unless it's been treated with coldness for a while.
Be kind to others, especially those with big hearts. Don't take them for granted, because often a cold-hearted person was once a person who cared too much for someone who cared very little or nothing at all.