Happiness and love, in the highest sense, could be considered congruent. That is, if you have love you have happiness, likewise, if you have happiness you also have love.
Love, being total unconditional acceptance of the moment, is a state without desire, without an existential imperative, without an identity. No identity can be owned, because all identity is transient, as are all phenomena, and thus ownership necessarily implies a desire that transient phenomena remain fixed, without change. Therein lies the problem.
Ultimate happiness is to be without desire. Essentially it is none other than the very state that is apparent, albeit momentarily, whenever a desire is fulfilled. Though it is not caused by fulfilment, but rather, by the cessation of desire. Put another way, to own desire is to experience unhappiness.
Interestingly, self-realisation leads to the gradual cessation of desire, in the most exitential sense. Seeing the insubstantial and transient nature of all phenomena allows grasping at identity to cease, and with it, the ownership of any desires associated with that identity.
Most peculiarly, there is nothing that can be done to stop desire, because to pursue such a course is simply to desire to stop desiring! You cannot win. Yet, to the degree this conundrum is clearly seen, desire simply ceases of its own accord.
Now you may say that such a state sounds profoundly unfulfilling, boring and drab. Such a contention can only arise, however, in the presence of confusion about the nature of self. The very fulfilment that you seek, is that which presents in the absence of desire! It is grasping at an identity, taking desire as your own, that takes you away from the very thing you crave.
From the perspective of compassion, ultimate happiness is also congruent with selfless love. Paradoxically, absent the tangle of identity, peace and empathy will move unhindered toward suffering and pain.