"A man's delight in looking forward to and hoping for some particular satisfaction is a part of the pleasure flowing out of it, enjoyed in advance. But this is afterward deducted, for the more we look forward to anything the less we enjoy it when it comes"

The quote indicates that when someone is delighted and anticipates an occasion or experience, it adds to their general pleasure of it. Nevertheless, the more anticipation and enjoyment there is, the more it eliminates from the actual enjoyment of the experience when it finally shows up. Simply put, extreme anticipation can result in frustration.