Happiness is really a state of Mind

Even though I have heard this many times before, I don't think I never really understood or realized it as much as I do now. As I start to think about it now, there been instances all along my life so far, where I was happy or sad just based on my thoughts. Based on how I was looking at things. I don't know what made be oblivious to that fact those times and what made me realize it now.