“To” is a preposition, while “too” is an adverb. Put simply, a preposition describes a relationship between two other words in a phrase or sentence, while an adverb modifies a verb, an adjective, or another adverb.

"Too" and "to" are easy to confuse because they sound identical. "Too" has two meanings: (1) "Too" means "as well." For example: Your eye is swollen. Your lip is swollen too. (2) "Too" conveys the idea of "in excess."

If the adverb too is followed by an adjective + to infinitive, we expand the sentence into two clauses, the first containing so and the second containing that. If the sentence containing too…to is in the affirmative, the sentence containing so…that will be in the negative.

With considerably fewer meanings and contexts than “to”, “too” is easier to remember. Always used as an adverb, “too”, may either be used to describe an excessive amount or more than necessary – in this case, it is placed before other adjectives and adverbs. “Too” is also a perfect synonym for “also”.

In the first instance you gave, the word "too" interrupts the sentence, "I want some pizza." Thus, "too" must be separated from the sentence - "I, too, want some pizza." But if the "too" comes after the sentence, there is no requirement for a comma (it is optional) because now it does not interrupt it, rather is mentioned like an afterthought.