You can train yourself to be happy without any reason by living in the moment. Focus on appreciating the little things, such as the birds singing outside, your morning cup of coffee, the aroma of each season and spending time with family. These are all fantastic things when you think about it, and you can always find something for which to be thankful even if you are someone who doesn't have very much in life.
Also, spirituality is something that many people find to be helpful in their life. If you can find joy and peace in the connection, you have with your sense of spirituality, and then you have done something remarkable for yourself. Wake-up every morning and remind yourself that your life matters and you have a purpose that is bigger than you.