Over the past many years, scientists have been studying the neurological and physiological effects of meditation and other mindfulness practices. This article from Kripalu Center for Yoga and Health summarizes some of the recent studies that focus on the benefits of lovingkindness practice in particular. Read the article for more nuance (and you can find a somewhat more academic article with more extensive citations here) but a quick summary of these benefits includes:

• Reduces the stress response
• Reduces inflammation
• Builds personal resources:  cognitive, emotional and physical resiliency
• Increases the experience of positive emotions, including love, joy, gratitude, contentment, hope, pride, interest, amusement, and awe
• Changes the brain in positive ways, most especially the insula and the temporal parietal juncture (TPJ) areas associated with empathy and the ability to attune to others.
• Increases a sense of social connectedness, even with strangers.

For these benefits to occur, it is important to practice at least a few minutes of lovingkindness a day. It’s not enough to do so only once a week or so.