My Dog has been acting rather strange what should i do?

during the day he acts like his normal self, happy, energetic, attentive and what not, but at night he acts different. usually when we feed him he cant wait to just grab the food outta of our hands, he would jump for it, and what not ( he loves to eat) but after i feed him what he normally eats, he doesnt give off the normal reaction, he looks at it now, and just sits and watches the other dog it. until the point where my other dog feels uncomfortable and will also stop eating. if left alone they both wont eat, the dog with the problems (Marley is his name) just faces off with my other dog (lucky), Lucky sometimes begin to bark at Marley and marley just looks at him. One Time i observed marley and when he got close to Lucky it seemed as if he were gonna be hostile to the other dog. i have to wait outside so that Lucky can finish his food in peace and i have to pet marley and give him a treat or something to try to get him to eat, what could be wrong? lately it seems like if he were upset about something.