I have came across few things (like about Allah/God existence, location, why, how, and many things about it) and I am always told that its not good thing to talk about it or its forbidden. Isn't there any reason why?
One can ask any question about anything in Islam. But what is discouraged is thinking about essence of God. And the reason mentioned is that the essence of God is beyond understanding of human and can lead to perplexity.
Allah knows what is [presently] before them and what will be after them, but they do not encompass it in knowledge. http://tanzil.net/#20:110
In this verse is said that human cannot encompass the knowing God.
It is like trying to each a mountain by teeth. It is basically impossible and the only result is losing teeth. Or it is like one person trying to drink all waters of an ocean.
We can understand only creations of God and attributes of God and cannot understand what is essence of God itself.
Different hadith from Ahl al Bayt discourage thinking to essence of God and instead recommend thinking about attributes and creations of God.
In fact human does not have the ability to understand essence of God.
Please note this does not mean stopping knowing God. Knowing God is the best valuable and enjoyable knowledge and prayer human can have ever. The attributes and creations of God are unlimited sources of knowing God. But the discourage is only about thinking about what is essence of God.