
"watashi" is usually used either by female or while talking with elders or superior as a polite form. For male, they most likely use "ore" as a manly form. "boku" usually used by boys or the guys who has a polite upbringing. So, it's rather cute when you have a (grown up) guy friend who still address himself as a "boku" while talking casually.

actually watashi is used by both genders and is the 'polite' or business form of "I"...it is the slimmed down version of Watakushi which is SUPER polite "I". Boku, is the male lazy way of say "I", Ore is the SUPER male lazy way of saying "I"... Atashi is the female lazy way of saying "I". You would use Boku around people you are familiar or friendly with. He is happy that Makoto-san has increased their intimacy by using less formal speech (Keigo).
can someone explain why using "boku" instead of "watashi" is considered cute?