Primary Motivation in Becoming Christian?

I think the primary motivations in becoming Christian should be:

1. Because it's true.
2. Because it brings great joy.
3. Because it gives our lives meaning.

Perhaps too short, or missing - but, indeed, Christianity is the most joyous religion, with the most good news possible. It is, in the face of a world clearly imperfect and with many suffering people, the happiest, best news we can imagine. 

In fact, we know it is greater than we can imagine - surpassing our best hopes and dreams.

The Saviour commands us to love God with everything, and we start by imagining or understanding God as He is - the most loving, caring Father possible, overflowing with joy and happiness and the infinite beauty of Himself and creation that He desires to share with us.

To participate we, ultimately, only have to let-go of the darkness in our lives, the falsehoods, errors, and flaws that somehow bind us and are destructive to creation and joy. We need to repent those things and seek God, to love Him.

The threat of great suffering is real, as we should know even now from sufferings in-this-life, but the greatest will be the loss of that relationship, and joy with God - He is infinite, and eternal, and the sufferings of that loss from a Heavenly perspective are of a greater pain, a greater loss to God's creation, and so worse than we can possibly imagine now (just as the joys are also beyond our happiest imaginings and dreams).