The speaker begins with a simple idea: we use words without really knowing them until a dictionary corrects us. Then he turns that idea on "friend," a word he admits he has used carelessly. He lists what friendship has meant to him: people he thinks about often, aches for, puts before his own needs, and feels free to talk to.
Then comes a sour realisation. Even in the closest bond, there is still "I" and "you." The self he brings to a friend is only the version the friend wants, "not me in my entirety." The tone here is dismayed and a little lonely.
The ending softens. He advises never taking each other for granted and being grateful, even just that someone is out there, reachable. Friends, he suggests, are like dictionaries, something to reach for when we search for the meaning of life. The mood is reflective, honest, and quietly resigned rather than bitter.