Occasionally, squirrels appear to be smooching each other, but it's not necessarily for romantic purposes. Belding's ground squirrels will “kiss” when they meet an unfamiliar squirrel, by pressing their noses together. Each squirrel has scent glands around their mouths that produce unique secretions. This scent allows squirrels to identify close relatives.
Female ground squirrels live in tightly-knit groups and will support their relatives, even by putting themselves in danger. This might include warning calls that alert others to predators but draw attention to the calling squirrel. Kissing enables the squirrels to determine who is family and worth protecting so they don't risk their lives for non-relatives.
Other types of ground squirrels may also use kissing to identify relatives in order to prevent inbreeding.
More about squirrels:
Maybe this gave rise to the expression: Kissin' kin.