Why not? Definition 1 of 3Pro Tip 1/3Positive Suggestion Implies that there is no reason against the suggestion and encourages agreement. 'We have some free time. Why not visit the museum?' Pro Tip 2/3Invitation to Join Often used when inviting someone to join an activity casually. 'I'm going for a walk. Why not come along?' Pro Tip 3/3Quick Agreement Shows quick and informal agreement to a proposal without much thought. 'Shall we order pizza?' 'Sure, why not?'