Studies have shown that helping others can decrease cortisol, the stress hormone, while increasing oxytocin, related to positive social interactions and generosity.

Additionally, acts of kindness can lower stress levels, promoting feelings of happiness, calm, inspiration, and generosity. These factors improve mental wellbeing, reduce depression and anxiety symptoms, and sustain mental health by fostering social interactions.

While doing an act of kindness can make a person feel good for a few minutes or hours, repeatedly engaging in acts of altruism can be a powerful tool to enhance mental health.

Much like exercising, the ‘high’ (of an altruistic act) will be most potent during and immediately after the act, which causes this pleasurable chemical experience.
The ‘high’ will dissipate—within minutes (in the case of oxytocin) or up to a week from when the chemicals were released in the body. To have continuously elevated moods that rely solely on this immediate effect, a person would have to do altruistic acts frequently to maintain the biochemical 'bump.

Neuroscience has found that by positively impacting others, people can feel an improvement in their personal well-being.

Source: Psycom

https://www.psycom.net/mental-health-wellbeing/how-helping-others-improves-mental-health