Can u be more specific? in what kind of situation ?
 

Simple answer: induce hormones that makes someone happy.
Long answer, it depends.
Generel ideas. Make them feel valuable. Work can make people happy. Being appreciated. Being complemented. Caring about their health. Showing interest in their hobbies. Accept them for who they are. Non-judgemental acceptance of the person’s identity. Instead judge the behavior of a person and not the person on itself. Reinforce positive qualities of a person. Show intention of wanting to help them. Actually help them. Try to understand the other person. Listening to the other person. Being truthful about their weaknesses. Being sparse with negative thoughts. There is only some much an ego can handle, especially if the other person has a fragile ego. Sharing thoughts focused on improvement goals instead of framing them as critique. Taking them out their anxious “comfort zone”. Take them shopping. Give them fresh air. Go to the forest: it has mind healing qualities. Take them to a sunny place. Sun shine releases endorphines in the body. Feed them chocolate: fat, sugar, and theobromine generally induce positive feelings. Let them watch laughing videos or comedy shows. Fend of negative people, sad television shows, depressing books, etc. Let them write their thoughts to clear lasting frustrations and provide insight in the vicious circle of negativity there in. Suggest to them to follow a therapy. Cognitive behavioral therapy improves happiness or decreases unhappiness. Mindfulness therapy reduces stress and improves health and happiness. Take them with you when you go sporting. Take them with you while running: this can turn into a good addiction and by running adrenaline and endorphines are released in the body reducing pain and giving positive feelings. 
I could go on for a while…