"Don't look a gift horse in the mouth" is a phrase that is used when talking about someone/people that look for the negative when he/she/they receive an unexpected positive.

To the people that normally over-examine the motivation of the person/party that is providing the surprise, or to the people that look for the flaws when receiving an unexpected surprise, you can say "Don't look a gift horse in the mouth."  The analogy is that the receiver, who has received a horse as a present, starts examining the small details, like the condition of the horse's gums and teeth, instead of being grateful for receiving the horse in the first place.

When something good occurs, and you did not expect it, then you may wonder why this has happened, or suspect that there was something wrong in some way.  If you receive a present from someone, you may suspect that the person who gave you the present has a sinister motive.

In addition, you could have been lucky, and due to circumstance, you may have been in the right place at the right time.  That may have resulted in good fortune for you, or your company.  Again, "Don't look a gift horse in the mouth" may be used.  Japan is having difficulty, and the Korean economy is benefiting.  Perhaps, Korea shouldn't look a "gift horse in the mouth."

So this holiday season, if you receive an unexpected present, then The Lost Seoul advises that you not look for ulterior motives. "Don't look a gift horse in the mouth." 

Merry Christmas...or is it Happy Christmas?