Ram Navami - Why Does God Descend?

On the ninth day of the Chaitra month, when the Lord manifested in His four-armed form, Mother Kausalya pleaded:

तजहु तात यह रूपा |
कीजय सिसुलीला अति प्रियसीला यह सुख परम अनूपा ||

tajahu tāta yaha rupā
Kijai sisulilā ati priyasilā yaha sukha parama anupā || 
(Rāmcharitmānas)

“Please give up this form, O Lord, and begin your childish sport, which is so dear to a mother’s heart. The joy such pastimes bring to me is solely unparalleled.”

At her request, the all-pervasive Lord, at His own will, turned into baby Shree Ram and began to cry.  Mother Kausalya’s joy knew no bounds as she took Him into her lap. She was blessed to feed Him, bathe Him, and rock Him to sleep. The Purushotham (Supreme Divine Personality,) who measured the three worlds with three steps in Vamana avatar, allowed Kausalya to teach him how to walk.  The Sarva Vidyā Pravartak (the originator of knowledge) willingly became a disciple of Vasishta Maharishi. Why does He perform these leelas (pastimes?) Or for that matter, why does He descend?

There are several reasons for His descension, but the most important of all is, as Vyas Bhagavan says: 
भवेऽस्मिन्क्लिश्यमानानामविद्याकामकर्मभिः
श्रवणस्मरणार्हाणि करिष्यन्निति केचन
bhave'sminkliśyamānānāmavidyākāmakarmabhiḥ
ṣravaṇasmaraṇāhāṇi kariṣyanniti kechana (Bhagavatam - 1.8.35)

“You descend for the welfare of the suffering souls, giving them a chance to relish the sweetness of devotion by hearing, remembering, and worshiping you."

Out of His causeless mercy and benevolence, He descends in His loving form to nurture love in our hearts. He makes one soul his father, another mother, another brother, and another friend, yet another servant or beloved. Hiding His almightiness and bound by the love of these blessed souls, He displays amazing leelas, which become a basis for our bhakti (devotion.) Hence, Saint Tulsidas, states:

राम एक तापस तिय तारी, कोटि खल कुमति सुदारी
rām eka tāpasa tiya tārī, nāma koṭi khala kumati sudhārī 

“In his descension period, Lord Ram helped only one Ahalya. However, since then, by chanting the divine name ‘Ram,’ billion fallen souls have elevated themselves.”

Therefore, when we contemplate on His name, form, virtues, pastimes, abode, and His saints, the joy we attain is boundless. Why is that so? It is because He is an ocean of infinite bliss, and we souls are his tiny parts. Therefore, we are naturally drawn to this bliss of the sweet Supreme Lord!