Pausing at a small, everyday moment, the speaker notices how a friend's face lights up when she walks over to say hello. He seems to go from stressed to glad almost instantly, and she realizes the smile is because of her. That discovery fills her with warmth.
What stands out is how much the speaker's self-worth is tied up in this friendship. She calls him the nicest boy alive, and if someone like that sees her as a friend, she thinks maybe she isn't so bad after all. Under the happiness there's a hint of insecurity, a quiet doubt about herself that his smile helps soothe. She also seems pleased that he started the conversation first, which suggests the friendship is growing in both directions.
The mood is sweet, simple, and openhearted. The plain, direct language and repeated "glad" give it the feel of a diary entry, honest and a little shy, about the joy of feeling noticed and valued.