Often one becomes very serious in the path of yoga and Sadhna, whereas Mother says the contrary. How can one cultivate an attitude of cheerfulness☺️ in oneself , in any kind of work which one does and in all kinds of relationships one comes across?

That is quite a correct observations. Seriousness should be only with regard to the goal and for this one needs to cultivate sincerity. But the more one comes in contact with the Divine Presence within, the more one experiences a causeless inner peace and delight leaving no room for anything but cheerfulness. The problem is that we take ourselves too seriously as if everything depended upon our efforts. Even sadhana is pursued for oneself, to achieve something, to become a yogi, or with some spiritual ambition, to have spiritual experiences, for one’s own mukti or personal transformation. This ends up building a self-centered state which takes away the inner joy.

The right movement is to become wide, to learn to see the Divine in everyone and everything, to put one’s effort not for oneself but for Her Will to be fulfilled in oneself and in all. It is to rely mainly on surrender to Her Grace while doing some self-enabling sadhana. 

Affectionately,

Alok Da