This Psalm begins by telling us how good it is to praise the LORD. We will get to the reasons why it is good to praise Him in the verses that follow. The psalmist is just reminding us how good, pleasant and fitting it is to do so.
We have already noticed in previous Psalms the benefits of praising God in light of His power. God is our power source through prayer and praise. As we praise Him, we express our love to Him and love others.
This praise is "pleasant and comely", that is, it is "pleasant and fitting". It should not be a burden to praise God. We do it out of appreciation for the good He has done in our lives as well as the world around us. It should not be a struggle for us to praise Him, but something that just comes out of us naturally because of our gratitude for what He has done in our lives.
 The LORD doth build up Jerusalem: He gathereth together the outcasts of Israel.
What does it mean to "build up Jerusalem"? Some scholars argue that this verse refers to the physical rebuilding of Jerusalem after the Babylonian captivity. Thiswould make sense as a historical application to this verse. God will one day restore and return the people of Israel to the land.
By way of present application, remember in Psalm 122:6 we read, Pray for the peace of Jerusalem: they shall prosper that love thee. Jerusalem is the city Jesus will rule the world from when He returns. This does not mean that all believers will one day live in Jerusalem. However, when the Bible says, "build up Jerusalem", think of it as building up all believers to learn to trust God with their lives. God is working in this world to get believers to trust in Him. God wants spiritual healing in the lives of all people.