"It's difficult to say 'no' to Syria strike."
Syria's president attacked his own people (civilians) days before the US attacked and now, it seems, Johnson might feel as if his involvement is mandatory (unavoidable).
Why, you ask... Because if the US asks for the UK's help, it'll be very difficult, nay impossible, to refuse. Whether that be a missile strike, submarine assistance or any other kind.
This all might happen without the Commons vote, because the law doesn't require the government to win it in case of launching a military action.