The third reason is scent. They identify colony members largely by smell, so licking and sniffing is partly them learning who you are, which is why scent bonding works: put a small piece of fabric that smells of you in their pouch or sleeping area and your scent becomes part of the furniture.
Four months in with a glider that climbs onto your hand voluntarily is good progress. Keep handling in the evening when they are actually awake.