Why did the Jewish believers in Acts share everything so eagerly?
They shared eagerly because they believed the Kingdom was just over the horizon. Expecting it soon, they saw no need for houses, lands, or personal wealth, since the coming Kingdom would have no poverty, no poor, and no need for private possessions.
Les explains that the Jewish believers in Acts 2 and 3 put their material goods into a common kitty and had everything common. He points out the reason they did that with such exuberance: they thought the Kingdom was just over the horizon. Who would need houses and lands when the Kingdom would come in? Because there would be no poverty, no poor, and no need for personal wealth. In the surrounding context, this follows Paul's mention that the Jerusalem leaders only asked him and Barnabas to remember the poor, which Paul was forward to do.