I think being left behind is one of the worst feelings in the world, and it can sneak up on you without warning. It can come from being physically left behind, like when a friend or family member moves or when you feel far away from someone you care about. It can be an emotional thing when you feel like everyone else has something you wish you had or when discontentment creeps in. It can be spiritual in nature when you wonder what God's purpose for your life is, or if you are in His will. And when it stems from all three areas all at once... it just makes you feel unimportant and unmotivated and untouchable. Even by God.
But I guess it's in the left behind times that you need to reach out to God the most, and trust that He knows what you are going through, that He cares what you are going through, and that He can work out what you are going through for His glory. I guess that is what faith is all about.