Actually, your friend is correct. Depending on the situation, this may be a sign that your seel is establishing his territory or attempting to court a mate. (The reason why it’s been occurring more frequently is likely because he’s come to the age where he can have either—territory or a mate, that is.) This is especially true if you’ve gotten a new pokémon neighbor or a new teammate, although in some cases, some seel may simply be trying to react to their own trainers.
The next time it happens, pay close attention to both your seel’s expression and what he’s focusing on. If he looks agitated or emits a series of loud barks and growls at the same time, he may be trying to drive away a rival. If the slapping occurs quickly and it’s more of a playful bark accompanied by a soft whine, he’s trying to attract a mate.