"We don't all say 'I care' the same way."
This is probably the simplest and most important thing I've ever learned, and has been the most helpful to me in understanding people that I care about who are different than I am.
You know the way you say "I love you." Whether that's in saying it, or doing nice things for someone, or making time for someone in your day... you have a unique way that you feel the most passionate about communicating your affection for someone. We want them to know, right? We want our friends and our loved ones to know how much we care about them and so we communicate that love in the way that makes the most sense to us.
But... what if they don't communicate the same way back? What if you say "you mean the world to me" and all they can muster up is "Thanks"? Does that necessarily mean that they don't care about you, too? I've learned that the answer to that question can often be "No." Maybe they're just someone who has a really hard time opening their mouth and saying "You mean the world to me, too."... but doing something nice for you is right up their alley.
If we spend all our time waiting for them to respond in the same way we offered our affection, sometimes we miss the equally heartfelt ways in which they are trying to say "I care about you, too." We mistake a differently-expressed love for indifference... and that is just about the saddest thing in the world. To miss that is tragic.