In order to answer such questions, we first need to make sure that we understand that we are speaking from an inherently and necessarily human perspective. That is, we must understand that it is us humans who are asking these questions, and again it will be us humans who will try to come up with reasonable and satisfying answers within our limits. In other words, we cannot read the mind of God and find the correct answer.
This is a general principle we should adhere to when dealing with matters of religion. Our faith should be based upon the reality we observe in this world. In fact, the word faith is a misnomer in English as the translation of the word iman in Arabic, which originally refers to a certainty derived from an evidenced conclusion.
So, when a mumin, or believer, says “I witness that God exists”, he is basing that conclusion on his observance of the world he lives in. He is essentially saying, “I have intensively looked around and studied things and found that the maker of this existence is not in it; thus, there must be a transcendental Being, who is All Powerful. Otherwise, the very existence that I studied and happen to be a part of cannot be explained and accounted for.” This is what tawhid is in a nutshell.
Going back to the question at hand, we must realize that we will answer it as much as we can from a human perspective. This does not mean it is going to be wrong, but rather it will be limited and relative and will have to depend on our reasoning ability as we used it above – gathering and interpreting the data in the world we live in. The answer is open to suggestions and reinterpretation as needs arise. Academically speaking, it will always be under construction as new arguments are brought forth. Some better versions of the answer we are going to offer here are bound to appear in the future as other intelligent human beings continue to think on this matter.
Therefore, one possible answer might be formulated like this: I look around and try to see if I, as a created being, might be of any use to the All-Powerful Creator. Here is one behavioral pattern among people, which can be used as a measuring stick to judge the reality of our original question with. For instance, farmers in some parts of the world choose to have as many children as possible to help take care of their livestock and fields that support their livelihood.
Is my creation in any form or shape similar to this? That is, am I somehow working for this All Powerful Creator? Am I doing something for Him? Something that He Himself could not do? The answer is a resounding no – not because someone else says so, but because I know so. I can say with certainty that I have not found anything in me and in my doings in this world that can be of any practical use for a Creator.
Not only am I not doing Him any favors, but I am totally dependent on Him –– from my existence to all my other endless needs and desires. If this is the case, as I have personally concluded with hard evidence, then what can be the reason He is doing all this for me?
One reason that comes to mind is that He is doing all this out of His love and compassion. He must be, as I conclude within my human reasoning of the above evidence, very merciful towards me. He does not expect me to work for him in return, but still He rewards me with my existence and everything else I am given. Who in this world would do such a thing for anybody else? Hardly anybody.
Thus goes the reasoning process in these questions that have something to do with a transcendental Creator. Look at the evidence and draw conclusions. Of course, the divine guidance that is sent down to us through His prophets is of indispensible necessity in tackling such existential questions.