Dogen's zazengi says
Zazen is the dharma gate of great ease and joy
What does the term "joy" mean here?
And if I can feel something like that without zazen, should I cultivate it? As well as learning meditation, I mean.
When there is joy, when a person is full of happiness and joy, there is little chance for ‘Uddhacca’ & ‘Kukkucca’ to arise. These are two of the hinderances mentioned in the scriptures. Restlessness (uddhacca) is agitation or excitement, which drives the mind from thought to thought. Worry (kukkucca) is remorse over past mistakes and anxiety about their possible undesired consequences. When one's mind is possessed and overpowered by restlessness and remorse, one cannot properly see things in a clear way. It is compared to when water is stirred by the wind, agitated, swaying and producing waves, a man is unable to see the image of his own face. Buddhists are taught to diffuse the Four Divine States of Mind: loving-kindness, compassion, sympathetic joy and equanimity towards all sentient beings, as this brings about a Joyous State of mind.
Merits, through ‘Daana’, ‘Sila’ & ‘Bhavana’ bring you joy. …. As the Buddha preached:
“Don’t ignore even a small merit thinking that it will not come to you! It is in fact with each drop of water that a pot is filled. Therefore, a wise person will collect all merits even if it is by bit by bit. In this way, his life will full with merits at the end.”
A joyous state of mind is important when learning to meditate. As the Buddha once explained:
“If the thought should occur to you that, when defiling mental qualities are abandoned and bright mental qualities have grown, and one enters & remains in the culmination & abundance of discernment, having known & realized it for oneself in the here & now, one’s abiding is stressful/painful, you should not see it in that way. When defiling mental qualities are abandoned and bright mental qualities have grown, and one enters & remains in the culmination & abundance of discernment, having known & realized it for oneself in the here & now, there is joy, rapture, serenity, mindfulness, alertness, and a pleasant/happy abiding.” — DN 9
The Pali scriptures describe two kinds of joy: (i) worldly/sensual joy; and (ii) dhammic or Zen joy.
Sensual joy is like joy from sex or drugs. The more this joy is indulged in, the more it harms a person via addiction & diminishing pleasure. These sources of 'joy' can cease to bring joy & instead bring pain (such as the torturous craving & withdrawal symptoms of drug addiction).
Where as Zen joy is completely harmless & always satisfying. The more Zen joy is cultivated, the greater the happiness (from generosity to harmlessness to jhana/Zen until perfect Nirvana).
Therefore, if you can feel joy without zazen, it does not necessarily mean you should cultivate it because some sources of joy (such as drugs or wrong sex) are very harmful (poisonous).