You should not doubt him.
I do not doubt his heart. Only the reach of his arm.
Why should Merry be left behind? He has much cause to go to war as you do. Why can he not fight for those he loves?
You know as little of war as that Hobbit. When the fear takes him, and the blood, and the screams, and the horrors of battle take hold, do you think he would stand and fight? He would flee. And it would be right to do so. War is the province of Men, Éowyn.