Hi, As aforementioned, do in this sense is used for emphasis. However, here are a few words of warning. Firstly, you will find 'do' used like this in conversational English, but you should not use it in written or formal English. Secondly, this is not the normal use of the verb 'to do', as I'm sure you've learnt previously, do normally has a completely different use.

As Ya-hsein said, DO (and DOES and DID) are used to emphasise and stress what you say. If I ask "Do you love your country", you can say, "Yes, I love my country" which is just a statement. However, if you answer m saying "Yes, I DO love my country", that tells me that you really love your country and that you mean it. You can answer "I wrote a letter to Santa" or "I DID write a letter to Santa". The second one shows that you specifically want to mention the fact that you wrote to Santa. Similarly "He DOES always love me" has more power and emphasis than "He loves me" which is just a statement and does not really show how much he loves. Hope this helps.

' do ' is used to emphasize here .

"I do have very good reason to be pissed off with you" is the same as "Of course I have very good reasons to be pissed off with you"