The legendary coach of the Pittsburgh Steelers - Chuck Noll - would tell his players this: "If I have to motivate you, I will fire you!" What this meant was motivation that lasts comes from within a person and not from outside influences.
There will always be the "Negative Nellies" as I call them and they are in every group, every company and every organization. And I will be darned if they don't always find each other. These people do nothing but tear things down. They will never build something up.
My suggestions are to find what motivates you from within and build on that. The second is when you run into the negative people, don't let them bring you down. Their negativity is all on them, and they only succeed when they infect others.