I'll echo what Deja and others have said and throw this out there:
To some, the greatest euphoria is the feeling of ecstasy caused by their brain being flooded with serotonin. For some others, it's dopamine. For others, it's divine revelations.
In any case, we don't all feel euphoric all the time partly because each person's idea of euphoria is at least a little bit different. In any case, though, it's a temporary state because it involves an imbalance of some kind, be it brain chemicals, selective sensory awareness, neuron activity, or whatever. It's one end of a spectrum, or one extreme of a system.
The nature of any system tends towards equilibrium. I think we are born in a certain euphoria, that is the ignorance and naivety of childhood. As we rapidly absorb information, and as our bodies stop growing, we reach a state of mental and physical balance or middle ground. It's just more useful to exist in such a state, because it allows you to perceive and judge experiences more accurately, by weighing the pros and cons and plan for potential future happiness. If we lived in a constant euphoria, our decision-making process would be radically skewed, even things we currently love to do would seem like a chore, and it would be very difficult to motivate anyone to do anything productive.
Think about this: if your mind was in constant euphoria, existing at peak happiness all the time, what incentive would you have to form interpersonal relationships? Nobody would have anything to gain, so nobody would bother to have sex, deal with the difficulties of raising children, and very soon the species would go extinct.