In other words, material happiness is relative. Suppose, for example, someone is drowning, struggling very hard, but suddenly he is able to come up for air- how much relief that person must feel. That's material happiness. Or after a long, hot day, one feels a cooling breeze, or a long lost friend sends a letter, or one eats a delicious meal after feeling very hungry...that's material happiness. Material happiness is dependent upon external circumstances.
Similarly, persons engrossed in the bodily conception of life, derive their happiness from external, yet temporary things such as a spouse, friends or whatever else is available to render relief from the pangs of material existence. Yet, as soon as there is some disturbance or break to that personal sense gratification, they feel morose and frustrated. As a result, they may try to cut that relationship off, fishing for any excuse, and seek happiness elsewhere, not knowing that the dissatisfaction they feel is the flickering nature of material happiness.
Spiritual happiness, on the other hand, has no cause and is therefore independent. It's the ever existing nature of our souls; we just have to tap into it. Dorothy, in "The Wizard of Oz", literally tapped her heels together to discover what she'd been running after, similar to her companions, who each in turn desired courage, love, and intelligence. They discovered they already possessed within themselves those things they'd been searching for.
That's why spiritual happiness is so nice. Since it's part of the ever-existing nature of the soul (sat-cit-ananda), it's portable. We can take it with us wherever we are, wherever we go... even to our next life and in any situation. In other words, our happiness doesn't depend on anybody or anything in the material world, whereas material happiness, as described above, is just the opposite.
Spiritual happiness, therefore, means no more material hankering. We feel completely satisfied with whatever condition of life we find ourselves in. Thus, we feel no more inclination to adjust our external position in order to counteract the inevitable distresses of material life. This ultimately means that we'll have no more changing of bodies; we become liberated from the cycle of repeated births and deaths which was the usual result from hankering for material pleasures.
Krishna describes the source and cycle of this hankering in Gita 2.63 which begins with the desire to be separate from the Lord, to become an enjoyer, contemplating the objects of the senses in hopes of happiness: "While contemplating the objects of the senses, a person develops attachment for them, and from such attachment lust develops, and from lust anger arises. From anger, complete delusion arises, and from delusion bewilderment of memory. When memory is bewildered, intelligence is lost, and when intelligence is lost one falls down again into the material pool."
But by regaining our portable, spiritual happiness, namely our loving relationship with Krishna, we can become free from this cycle that fails to satisfy our innermost needs and instead, equal towards the external happiness and distress that continuously comes and goes, find the power to take the advice given by Lord Krishna in Bg 2.14:
"The nonpermanent appearance of happiness and distress, and their disappearance in due course, are like the appearance and disappearance of winter and summer seasons. They arise from sense perception... and one must learn to tolerate them without being disturbed."
Gradually, such steadiness in our duties elevates us from the muck of material existence:
"O best among men [Arjuna], the person who is not disturbed by happiness and distress and is steady in both is certainly eligible for liberation."--Bg 2.15
Therefore it is essential, first and foremost, to cultivate our spiritual life and inner happiness, our reconnection with Lord Krishna. By His mercy, we can understand what actual happiness is, and how, as a spiritual being, it is an integral part of our true nature.