Why do I sweat in my armpits and chest so much when I walk to work, even though I am not overweight?

well it is warm outside here in Los Angeles, but even then, I walk about 2 miles to work in the morning and 2 miles back. so even when its hot during the day, the mornings and evenings are usually milder.

it makes me think that maybe I must be out of shape for sweating so much in my armpits and chest.
Update: Kyle, it doesn't happen in colder weather
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