It's true that a partner in love can make one happy, no doubt. That love can be liberating, deep and precious. The downside, is that, this happiness is not permanent or lasting. It will disappear again and one will experience suffering as a result.

What Buddhism teaches, is that if one would like to achieve a permanent happiness then one should not look for it in conditioned existence (samsara), because it cannot be found in a world that is impermanent. 

To achieve lasting and permanent happiness one must realize Nibbana, the unconditioned, unborn and uncaused state. 

Nibbana is not subject to incessant arising and falling away, thus it is stable, secure and permanent.