Question

Good or bad, well it all depends
A straightaway you're on that bends

Acting as a mood shift agent
Like a scent found foul or fragrant

If I'm pleasant, then I'm greeted
Seen ahead, then I'm defeated

The honoree unimparted
I can get this party started

What am I?

Answer 1

a surprise

lines 1-2: A surprise can be either "good or bad". The second line is a metaphor for an unexpected turn or "bend" in the road.

lines 3-4: A surprise can play a role in affecting one's mood, and is likened to the detection of a scent which can be perceived as either negative ("foul") or positive ("fragrant").

lines 5-6: A "pleasant" surprise is welcomed or "greeted". If one can foresee an event taking place, then it is no longer a surprise, and the would-be surprise is "defeated".

lines 7-8: These two lines are in reference to a surprise party in which "the honoree", the one for whom the party is being held, is kept in the dark beforehand.