Assumption, Assuming, Circular Reasoning

Any diseases including over-weight could be caused by anything therefore "assumption" is to be avoided.

This is where we are to discern the issue. There was a discussion somewhere about "What if Christians are guilty of circular reasoning?" This is called "begging the question in logic", the Latin name petitio principii, is an informal fallacy found in many attempts at logical arguments. An argument which begs the question is one in which a premise presupposes the conclusion in some way. Such an argument is valid in the sense in which logicians use that term, yet provides no reason at all to believe its conclusion.

Using knowledge without facts can easily lead us to a clearly false conclusion. Conclusions drawn in this manner are usually overgeneralizations. Often people will use "logic" and "argument from analogy" (relating things to novel other situations) to prove their case. To use an analogy:
1. Start with a target domain where you want to create new understanding.
2. Find a general matching domain where some things are similar to the target domain.
3. Find specific items from the matching domain.
4. Find related items in the target domain.
5. Transfer attributes from the matching domain to the target domain.

My beef of most abused analogy is relating human father to Godly Father.

So therefore, assuming reasons why a person is fat is sin. This is like Job's friends, who were obstacles to Job's relationship with God when they don't really know why things are happening to Job. Their lectures were based on logics, assumptions and reasoning. The problem was was that all their accusations were unfounded. He did not know Job or the tthings he had done. We must be careful about how we react to the problems or sins of others. We must not accuse others of things we are uninformed about.

Christians filled with the Holy Spirit do not make assumption and will not act like Job's friends, making bad theology based on what someone is facing when they don't know all of the facts.