"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.
What Girls & Guys Said
The rebels attacked civilians, and blamed government... if another country gets in the fight things will get worse.
Nothing will change until people realise what the interest of NATO, the US, UK, the Arab League and others supporting those who fight against Syria is, they only care about oil; and the media is guilty of it too.
The US isn't attacked thus has no ground to ask for the UK's help.
How about the put Boris Johnson on a warhead certainly that would make the U. K better