Something that is buoyant floats in water. Since floating is happier than sinking, buoyant also refers to things are fun and upbeat.
Someone with a buoyant personality is fun to be around, laughs a lot, smiles, and cheers other people up. Buoyant people are lively and lighthearted — the opposite of sad, depressed, and bummed out. Buoyant people are also called bubbly and cheerful, and it should help to remember that buoyant objects float — just like a buoyant person can seem like they're floating too (as in the expression "floating on air").