By the same token, if we seek out positive people, their pleasant demeanor starts to rub off on us. If we marry someone who builds us up, we want to build others up. If we seek out friends who rejoice with us in the good times, but comfort us in the bad times, we want to return the favour. If we choose books to read that have a message of hope, then we start to become hopeful. If we listen to music that inspires us, we will be uplifted.
So, feed your souls well. For what goes in, must come out.