Imagine someone holding a glass full of clean, fresh water and complaining about thirst. Likely you'd suggest they first take a sip from glass in their hand. Happiness is similar. Everyone wants to be happy, but not everyone knows how to recognize and stay with it; they're always looking for more. The search for happiness is lost when it becomes an insatiable pursuit for getting more.
The difference between two is like difference between savouring and lingering over sweetness and flavour of a mango, and quickly gobbling it up before eating next sweet. The pursuit becomes focus, rather than experience or satisfaction that comes from what we do have.
This endless pursuit for happiness can consume us for all of our lives. We may think that once we have more money, a relationship, or that perfect job we'll be happy, yet when we get there we find it's not what we'd hoped for, or we don't take time to really enjoy it.
There is always something more to be pursued, bought, owned, done, that we rarely enjoy what is in front of us. Even search for spirituality is pursued in this manner. People go from spiritual leader to leader searching for meaning, often going as far as India to find fulfilment.
The pattern is easily recognizable, and we can all fall into it with thoughts like "when I do...own...have...get...go to... I'll be happy", or "if only...would happen." But truth is once whatever is sought after is obtained, we're off looking for next thing. We rarely stop and simply enjoy what is happening right now or fully appreciate what we have.
Some believe this constant desire and pursuit for more is rooted in our biology — that it helped us to survive when we didn't have all conveniences that are available to us today. Some believe that this pursuit is rooted in a society that emphasizes consumerism, and another view is that it reflects an alienation from ourselves and one other.