What is happiness? Is it a chemical reaction? Is it feeling fulfilled in life? Is it looking forward to the future? I would say it is all of these things.
Chemical reactions in our brains fuel our experience of life, and they are highly complex. In the end, it is the amalgam of all the reactions that translate into how we feel. That being said, our experiences in life, positive and negative, affect these reactions. For example, leading a harmonious life, having success in work and love, getting proper rest, enjoyment of family and friendships, maintaining a balanced diet and exercising, each one affects us and our chemistry. The psychological relates with how to do all of these things well, that then gives us that feeling of happiness.
Sometimes, however, traumas, genetic predisposition, and negative patterns from earlier experiences make it very hard. Psychotherapy, along with medications (as needed) can address all of these issues
to help optimize your life.