You use too after mentioning another person, thing, or aspect that a previous statement applies to or includes.

You use too after adding a piece of information or a comment to a statement, in order to emphasize that it is surprising or important.

You use too in order to indicate that there is a greater amount or degree of something than is desirable, necessary, or acceptable.

You use too with a negative to make what you are saying sound less forceful or more polite or cautious.

You use all too or only too to emphasize that something happens to a greater extent or degree than is good or desirable.