References – SN 55.40: Nandiya the Sakyan 
“It has been a few years since my last retreat. May I request a reminder on how to practice metta meditation on oneself? ... I seem to have forgotten.” Answered by Ajahn Pasanno. [Goodwill] // [Meditation retreats] [Happiness] [Concentration]
Reference: Amaravati Chanting Book, p. 41: “May I abide in well-being.”
Sutta: Snp 1.8: Metta Sutta (Amaravati Chanting Book, p. 37).
Suttas: AN 11.1, AN 11.12, and SN 55.40: Causal chains linking gladness to samādhi.
Abhayagiri Monastic Retreat 2013, Session 4, Excerpt 4
“I find it very difficult to stay with the breath. After 15-20 minutes, the mind will be very bored with the breath and cannot stay with it. Could you advise what is a good meditation for an easily restless mind?” Answered by Ajahn Pasanno. [Mindfulness of breathing] [Restlessness and worry] // [Attitude] [Desire] [Meditation/Techniques] [Investigation of states] [Gladdening the mind] [Happiness]
Sutta: MN 118: Ānāpānasati Sutta
Suttas: AN 11.1, AN 11.12, and SN 55.40 illustrate the causal chain between happiness and samādhi.