Why? Because it was fun for him to witness Seward's reactions to the supernatural, probably. He secretly loved that reaction would become clutching the flowers in despair and horror, of course.

LOL, he's such a meanie! I tend to explain it away as being that he needs a miner's canary, and Seward is nothing if not a miner's canary for vampires. (If there's a vampire anywhere nearby he will faint, sob, and/or fall into its clutches, giving Van Helsing warning.) But I'm not sure any logic works there, except maybe that he hoped Seward would clutch at him instead of the flowers.