Because she stole his mushrooms?
If the little Maquis girl had grazed her front up against his multi-chromatic tuxedo and fluttered something, like maybe her eye lashes possibly, it's autonomic that Neelix would have completely opened up his larder to whatever... Integrity?
This fuzzy stubby man once used Kim's blood on his knuckles as an aphrodisiac!
In this case, Seska wanted Chakotay to join her conspiracy and enjoy her forbidden fruit, dirty as it sounds, at Neelix's expense! Seska failed unfortunately because Chakotay had too much integrity to steal food from the ships stores.
Neelix on the other hand is a no good dirty drug dealer.