Assalamo Alaykum. Is it allowed to say dear Allah?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

Apparently, there is nothing in Sharee'ah that prevents one from addressing Allaah The Exalted with the expression "dear Allaah". If the one who says it means by it "my beloved", then there is nothing preventing that either, because Allaah The Exalted is The Loved by His allies and He is loving to them as well. He says (what means): {… a people He will love and who will love Him.} [Quran 5:54]

Ibn Al-Qayyim said: “He, glory to Him, is qualified as having extreme force; however, He is also the Merciful, the Loving (Wadood), the one who shows affection to His slaves by His favors; the one who loves anyone who turns back to Him in repentance; and He is also the Loved (Wadood), i.e. the beloved. Al-Bukhari said in his Saheeh that Al-Wadood means "Al-Habeeb" (Beloved). The fact of the matter is that "Al-Wadood" refers to both senses: to His being loving to His allies and to His being loved by them. Thus, one of (the senses) is the basic meaning and the other is corollary to it. He is The Loving who loves His allies and is loved by them. Shuʻayb said as reported in the verse (which means): {"Indeed, my Lord is Merciful and Affectionate (Wadood)."} [Quran 11:90].” [At-Tibyaan fi Aqsaam Al-Quran]

Allaah Knows best.