Use 'buck up' to encourage someone feeling down to become more positive and energetic.

Emotion Boost

It's a casual phrase, not appropriate for formal situations like talking to a boss or in solemn events.

Not Too Serious

It's usually said to individuals, not groups, as a personal encouragement.

It implies that someone must make an effort and not rely on others or fate to improve a situation.

Use it to refer to specific actions or tasks that need attention, not vague situations.

It suggests self-initiated improvement, whether it’s in skills, behavior, or performance.