Commas before and after "therefore" but not before and after "also"

Consider the following sentences:

The shirt is pink and is, therefore, very stylish.

The shirt is pink and is also very stylish.

In the first sentence, I feel as though I need to use commas around the word "therefore." However, I don’t feel the need to use commas around "also" in the second sentence. This is strange to me because it seems that the two words are acting as the same part of speech in each sentence.

Is the punctuation in one of these sentences incorrect? Is there some subtle difference between the words "therefore" and "also" that explains why commas could be warranted in the first sentence but not in the second?