Hi Sharmel. My understanding is that 고맙다 is often used among friends to express sarcasm. Using the infinitive of a verb -다 takes some emotion away, so it's perfect when you want to be sarcastic. Interestingly, it can also be used in really serious situations when you want to express deep gratitude. In my case, I've only ever heard it used sarcastically, so... Hope this helps.

here are seven verb paradigms or speech levels in Korean, and each level has its own unique set of verb endings which are used to indicate the level of formality of a situation. Refer to https://en.wikipedia.org/wiki/Korean_speech_levels 1. 고맙습니다 is Hasipsio-che This conversational style is generally called either the "formal" or the "formal polite." This is a common style of speaking. A conversation with a stranger will generally start out in this style and gradually fade into more and more frequent haeyo-che. It is used between strangers at the start of a conversation among male co-workers by TV announcers to customers 2. 고마워요 is Haeyo-che This speech style is called the "polite" style in English. It is remarkable in that it is used both with higher level pronouns. It is used mainly: In Korean phrasebooks for foreigners. Between strangers, especially those older or of equal age. Between female co-workers or friends. By younger speakers as a less old-fashioned alternative to the hao-che. By men and women in Seoul as a less formal alternative to the hasipsio-che. 3. 고맙다 is Haera-che This conversational style is generally called the "plain" style. In writing and quoting, the plain style is the equivalent of the third person. Any other written style would feel like a first person account (that is, anything else would seem to be told in the main character's own voice). It is used: To close friends or relatives of similar age, and by adults to children. In impersonal writing (books, newspapers, and magazines) and indirect quotations ("She said that..."). In grammar books, to give examples. In some exclamations. 4. 고마워 is Hae-che This conversational style is called the "intimate" in English. It is a defective paradigm, meaning it lacks most of the expected conjugational forms. It is used Between close friends and relatives. When talking to children.