Getting your hands dirty makes you feel good.
According to research, putting your hands in soil puts you in contact with mood-enhancing soil bacteria called Mycobacterium vaccae. This bacteria triggers the release of serotonin in your brain. Serotonin is a mood-boosting happy chemical that also works as a natural antidepressant and strengthens the immune system.

Harvesting your own produce gives you a high.
Turns out that harvesting your own produce from your own garden provokes the release of dopamine. Dopamine is a hormone released in the reward center of your brain that is tied to a state of bliss. This harvesting dopamine release can be set off simply with the sight or smell of your fresh produce, as well as by the act of picking it. I can personally confirm that taking a bite of it then and there will only add to the bliss.

Focusing on the task at hand breeds contentment.
Putting your sole focus on the gardening in front of you makes you feel content. Being able to fully fixate on the immediate details of gardening reduces rumination and gives you a space to be fully present. This is a mindfulness practice and automatically eases stress levels. This will naturally improve your attention span elsewhere, too, whilst reducing the risk of anxiety and attention-deficit/hyperactivity disorder symptoms.

Physical activity gets your endorphins going.
Planting, pruning, weeding, digging, and raking—if you’ve ever tried them, you know how physical they can be. As such, gardening is a form of physical exercise. Exercise lowers your stress levels and boosts the production of endorphins, your brain’s feel-good neurotransmitters. We all know exercise is good for our bodies and minds—so why not exercise in the natural way gardening encourages you to?

https://www.psychologytoday.com/us/blog/the-path-to-passionate-happiness/202404/why-does-gardening-feel-so-good