Kindness & Gentleness
Undoubtedly, the heart leans towards those who are kind and gentle. It was said a long time ago: "Make your face happy and your words gentle, and you will be more beloved to the people than those who give them a gift". Likewise, it is a part of human nature to turn away from severity and harsh heartedness even if it were to come from (our Prophet who is) the best of Allaah the Most High's creatures.
Allaah the Most High says:
"...And had you been severe and harsh hearted, they would have broken away from you..."
The one calling to the religion of Islam and dealing with other people has the utmost need to gather the people around him, and cause their hearts to lean towards him so that they will listen to him. Adorning himself with kindness and gentleness helps to achieve that goal with the grace of Allaah.