To breakdown this line which is: 楽しみに + する+ ている- looking forward to it.
I'm aware that N+にする could mean 決定,変化 But this doesn't fit the translation of looking forward to it. or does it mean 目的 which indicates the purpose or intention behind the action
I was originally going to say that I could see two interpretations, but after thinking about it one can be discarded. The interpretation I considered, but realize must be wrong, is that “楽しみに” is an adverbial form of “楽しみな” and as such it would be similar to “いい子にする” and “好きにする” as in to simply do something in a forward-looking way.
The correct etymology is surely that “〜にする" can also mean “to regard as” or “to consider as” as in “I consider it something to look forward to.” the reason the other interpretation must be wrong is because of the object in say “誕生日を楽しみにしている”. If it were the adverb it would mean “I'm having my birthday in forward-to-looking-matter.” and I suppose it theoretically can mean that, but it means “I'm looking forward to my birthday.” or “I regard my birthday as something to look forward to.”
Other similar constructs are “必要とする” [to need, as in “to consider necessary”] or “〜とされている” [“is considered to be”]. “〜に" and “〜と” are largely interchangeable with “する” but fixed expressions use one over the other.
