Final answer:
Henry is referred to as a 'party pooper,' indicating he negatively affects party atmospheres, hence the party is kept a secret from him.
Step-by-step explanation:
The main reason for keeping the party a secret from Henry is that he is considered a 'party pooper.' This term suggests that he either does not enjoy parties or has a negative impact on the atmosphere of the event, possibly by being a downer or dampening the mood of other guests.
Consequently, the individuals planning the party prefer to keep it a secret from Henry to maintain a lively and enjoyable environment for everyone in attendance.