When the day of Pentecost had arrived, they were all together in one place. Suddenly a sound like that of a violent rushing wind came from heaven, and it filled the whole house where they were staying. And tongues, like flames of fire that were divided, appeared to them and rested on each one of them. Then they were all filled with the Holy Spirit and began to speak in different languages, as the Spirit gave them ability for speech. Acts 2.1-4 (HCSB)

To rest means to be placed, to cease work or movement. It also means to relax, dwell, be a model; this is, serve. Besides these things, the verb rest is also used to place hope, trust or confidence on or in; therefore, to impress someone.

When the Spirit rests upon a person, he becomes a model example of the Lord Jesus. He begins to demonstrate such a character, that all those around him are impressed by his behavior.

Example: When the Lord Jesus was brought before Pilate, he was impressed with the behavior of Jesus. He was being accused of something he had not done, yet He was not defending Himself, because He knew the One that was in Him was greater than everything and everyone.

For this reason, God wants everyone to be baptized with the Holy Spirit. This is not an option, but an imposition made by Him.