The great scholar Ibn ul-Qayyim (رحمه الله) mentions:
"There's nothing more beneficial for the heart than treating the people with kindness and wishing good for them. Because by treating the people in that way, then;
• if it is a stranger you will gain his love and affection,
• if it is a friend and beloved one you will maintain his companionship and love,
• and as for a hated enemy then you will extinguish his firebrand with your kindness and suffice against his evil."
He (رحمه الله) also mentions:
"The one who treats the people in a nice way and has good suspicion about them, his intention will be in a good condition, he will feel at ease, his heart will be well and Allah will protect him from any evil and calamities."
Translated by 'Abdurrahīm Ibn Muhammad Al-Hadhramī.
Published on Salafi-Dawah.com.
Source: Madarij-us-Salikin 2/511.
يقول ابن القيّم - رحمه الله
فليس للقلب أنفع من معاملة الناس باللطف وحب الخير لهم فإن معاملة الناس بذلك إما أجنبي فتكتسب مودّته ومحبته ، وإما صاحب وحبيب فتستديم صحبته ومودّته ، وإما عدوٌّ مبغض فتُطفئ بلطفك جمرته وتستكفي شره
وقال أيضا رحمه الله: ومن حمل الناس على المحامل الطيبة وأحسن الظنّ بهم سلمت نيته وانشرح صدره وعوفي قلبه وحفظه الله من السّوء والمكاره
2/511 مدارج السالكين