Joy.
It is defined as "a feeling of great pleasure and happiness".
It is not a destination.
We are so used to seeing people pretend to be happy constantly. It's almost as if when you're not feeling happy, you're failing. Happiness is not a level to attain to. Here's the bottom line: while happiness is a feeling, joy is a choice.
At some point during high school I fell into the trap of discerning happiness from joy. I got very caught up in trying to be happy - I would make decisions on what little of my life was within my control in an attempt to make myself (as well as people I cared about) happy. I studied hard. I joined extracurricular activities that weren't particularly meaningful to me because I thought they would lead to some long-term happiness.
It wasn't until the summer after my junior year of high school that I began to understand the concept of joy versus happiness. While I may not always be happy about circumstances of my life, I can always choose to seek joy from the God from whom all joy comes.
Please note: this message is not intended to promote the idea that everyone should just ignore hard things and "be happy". I am not under the impression that being happy is that simple. However, I understand that the concepts of happiness and joy, while often use daily interchangeably, are vastly different. While happiness is fleeting, joy is eternal. Seeking joy is a conscious choice. Sometimes, it may even mean choosing to find the good in horrible circumstances. At the very least, it means recognizing that our God is one of joyful celebration. Psalm 4:7 states that "You have filled my heart with greater joy", and I can't think of a scripture that does a better job of expressing what joy God instills in us.
In hard situations. In celebrations. Seek joy.