At a certain point in the first season of the show, it was revealed that Coulson died for a lot longer than the number of seconds he was led to believe he died for.
He actually underwent an experimental procedure named Project T.A.H.I.T.I, which was designed to resurrect a member of the Avengers in the event of their death using the blood of an alien from the Kree species. Nick Fury decided that Coulson was so important to the creation of the Avengers that his death warranted the use of the procedure too.
The reason that Coulson can't remember any of this is that the process of bringing people back to life also had the side-effect of implanting the Kree's "genetic memory" into them, sending the test subjects mostly insane and compelling them to carve strange alien symbols and diagrams into whatever they could get their hands on.
As such, a memory wipe became part of the procedure in order to attempt to remove the implanted "genetic memory" - however, unbeknownst to SHIELD, that only ever worked temporarily. After seeing how traumatic the overall process was, the person running the tests shut the project down - in an ironic twist, that would be Agent Phil Coulson himself. Whose memories of the project were deliberately wiped when he was bought back to life using the very project he had tried to stop. His memories of this period were replaced with - you guessed it - a relaxing holiday in Tahiti. It's such a magical place.
In short, Coulson was bought back to life by an experimental procedure that he personally disagreed with, which is why its use was kept secret from him.