There's a lot of wise words in this Psalm from David. It's very easy to become distracted by other people in this world. We see other people prospering and driving fancy cars and living expensive lifestyles, and we say, "Man. Why can't I have/drive/live like that?" And sometimes those thoughts consume us... and we lose sight on what is really important. David is saying here that we shouldn't put our focus in the people around us... but we need to be delighting in the Lord and committing to Him - and everything we need will be taken care of. It's a refreshing and wonderful thought that we need to remind ourselves of every now and again.
We can't love this world. We can't love stuff. We can't want to have somebody else's lifestyle... But we can and we NEED to be loving God and fully committed to Him and His Will for us. That is what is going to truly make us happy.