Psalm 100 has two parts (verses 1-3 and 4-5) and both follow the same outline: first there is a call to worship. Then follows a reason for worshiping.
Like Psalm 96, the call to worship is not just for God’s people, but for “all the earth.” For God so loved the world that he gave His son, that whoever trusted in Him might have eternal life.
The word translated “worship” at the beginning of verse 2 can also be translated as “serve” and from that notion, we sometimes call our church gathering a “worship service.” In pagan theology, worship to the gods involved seeing to their needs, thus people came to worship to provide for their god and thus “serve” Him. But Old Testament theology does not provide for such a notion. Our God needs nothing from us. Thus the call to “serve” the Lord is a call to submit to Him as a servant, to do His will. Verse 2 then has two calls: first to delightful obedience (“worship the Lord with gladness), then to assembly (come before Him with joyful song).
The psalmist gives two reasons for worship: He created us, and therefore, we belong to Him, and He is good to us, keeping His word to us constantly. He is not good to us because we worship. He is good to us first. And we worship because of it.