Tranquility is a necessary component for contentment. Tranquility is also the proximate cause of insight. This is generally why teaching concentration practices precede insight or vipassana practice. Only a calm mind can realize its true nature: radiant and pure.
Humans are blessed with breath; an ever-present biological function that acts as a conditioner for the body-mind system. Quality of breath directly influences quality of mind and body. When we are stressed or fearful, breath is fast, short, and shallow. Conversely, slow, long, gentle, deep breathing leads to cognitive-affective-somatic contentment and restfulness. You may have noticed when you feel agitated, if you put your attention on how breath is and gently slow in-breath and out-breath, anxiety and agitation subside.
Adding awareness or what is called “relaxed attention” on breath in a focused way calms the body-mind system. When we stay with breath long enough, calm leads to interest in the mind, and joyfulness in the heart and body. Eventually, the excitement gives way to a contentment, which arises from the direct experience of the mind knowing its own radiance and clarity. This is what the Buddha famously taught in the Ānāpānasati Sutta (find more information in my textbook on Buddhist psychology for clinicians.)
If radiance and clarity is the true nature of mind, why do we not experience these qualities of mind all the time? Primarily this is due to the presence of habitual thought-generated mental hindrances, such as craving, aversion, laziness/inertia, restlessness, and doubt, which grip conceptual mind and prevent it from realizing its own empty, luminous essence.
In concentration meditation we learn to stop feeding the hindrances by starving them. We train the mind to stay present with an object like breath, which naturally leads to calm, clear, and contented states of mind. Continually choosing over and over again, to turn away from distressful states of mind and turn toward the experience of breath eventually gives us the confidence, to turn the mind toward the hindrances, and stay present with these distressful states of mind to engage in the inquiry of vipassana meditation practice. You can learn more about this on the Groundless Ground Podcast Episode with Buddhist teacher Shaila Catherine.