It's hard to be positive all the time. All types of things happen. You lose your job. You get bad grades in school. Your spouse leaves you. Friends lie to you. There are just a million and one things that will beat down on you and make you feel miserable. I cannot even begin to give a reason why bad things happen. Maybe sometimes it's the devil attacking us. Maybe sometimes it's God trying to make us stronger. Maybe sometimes it's just life. I do not know.
What I do know is that we must always have hope in God. He made a promise to us and he does not break his promises to us. As long as we accept him and do separates ourselves from him through sin, then we shall spend eternity in His presence.