Not only are they emotionally inert themselves, they are an emotional drain on others. If we are around cheerless people for any time at all we feel our own liveliness being sucked away. The bloodsucker soon leaves everyone feeling anaemic. The cheerless person debilitates everyone else. Cheerfulness is so very important.
Scripture mentions it again and again.
It's obviously part and parcel of the Christian life. The truth is, no apostle ever pretended that we are made cheerful by looking around us. When the apostles looked out around them they saw an army of occupation. They saw grinding poverty. They saw betrayal at the hands of political leaders and religious leaders alike.