Whenever we wish for something to God, why should god provide us with what we ask?
What does god get by giving, whatever we wish for?
Does god fulfilling our wishes is also part of the plan of God?
First of all, we should know the right interpretation of the term "God". If by the word "God" you mean the ultimate reality, then according to various philosophies including Bhagavad Gita, that's called the "Self" (Atma). Atma is common among all beings.
Like how the same artist creates 'Jack & Jill', who love each other OR 'Tom & Jerry' who hate each other. Similarly, it's the same "Self", which creates everything, which interact with each other.
This also includes we praying for material desires and getting fulfilled by its respective deity.
"Why should god provide us with what we ask?"
God always fulfils one's desires. How soon -- is based on the intensity of obtaining it:
BG 4.12 - Acting desirous of result, those who worship the deities# here in this human world, the success happens quickly from those actions
# - worshipping deity means, acting in certain aspect
What we ask, is under God's (read "Self/Atma") supervision and when will it be fulfilled is also under God's supervision.
The intention of fulfilling the asker's desires -- is to strengthen the devotee's belief system in that aspect (i.e. deity). Because all aspects (or deities) lead towards the same supreme God.
BG 7.21 - Whichever form [of a deity], any devotee wants to worship with faith, I strengthen only that faith of them
e.g. If one desires wealth, then one acts in those direction which can gather wealth. In old times, it was termed as "praying Kubera". At core, both are same.
"What does god get by giving, whatever we wish for?"
Whatever we wish for, is due to helplessness of our nature.
But, God doesn't bind itself, to get something in return, upon fulfilling our desires. God runs the the world, completely disinterested.
BG 9.8, 9.9 - Keeping My nature (prakruti) under control, I create [& destroy] the world of beings again & again; which are helpless under the nature (prakruti); And O Arjuna, those actions (Karma-s) don't bind Me, as I am disinterested & detached to them
"Does god fulfilling our wishes is also part of plan of God ?"
As stated above, whatever we wish for is determined and whenever will it be fulfilled, is also determined. God is the originator of our [not so free] "Will".
Read this answer, about how the time cycle repeats and hence renders everything predetermined.
God gives everything you ask because he has everything. So when you have everything there is nothing happier than giving.
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Cosmos is in the constant state of "Thathaasthu". What even one aspires for the cosmos doesn't deny it.
God is the representation of that cosmos and has overflowing love for existence / life. Since we are a reflection of that whole (although we don't realize that), what ever "inner space" we carry becomes reality.
It's out of love that the cosmos says "Thathaasthu" without judgement.