This is the 11131st most frequent Korean word.
기분이다
"It's the mood" or "It's the vibe."
Here, '기분이다' is applied indirectly by describing the mood as a result of the described situation—in this case, being with friends.
친구들과 함께 있으면 즐거운 기분이 된다.
Being with friends makes me feel happy.
In this instance, '기분이다' conveys a spontaneous decision or an impulse arising from the speaker's mood or feelings.
기분이다, 먹고 싶은 음식을 다 먹어 보자.
I feel like it, so let's eat all the food we want.
In this sentence, '기분이다' is used to express one's current emotional state or mood by describing how the speaker feels.