>>913970281. Armpits are ticklish and otherwise sensitive.
2. Armpits have major blood vessels close to the skin, making them extremely warm. Hot, even.
Given this and the previous point, they are kind of a vital spot, which is why exposing them signals vulnerability, and vulnerabilities are exposed during moments of intimacy.
3. Armpits produce a bodily fluid (sweat), and smell (optionally, if you're into that).
4. Armpits are pretty close to other sensitive/erogenous areas such as neck and breasts.
Just from the top of my head; there may be more to it.
