Bangkok Post says
It is yet unknown the real reason behind his resignation as Mr Tej could not be reached for comments. But it is believed that Mr Tej is discouraged by the work of the government and his tough role in explaining to foreign communities about ongoing political turmoil in Thailand.
and The Nation
The sources said Tej, former permanent secretary for foreign affairs, reasoned his resignation that his wife, Pensri, is sick.
It is still unclear why he wished to resign.
Giving Tejs credentials and the first successes in calming down the Preah Vihear conflict with Cambodia, I hope he would stand by for returning to his post in a government of national unity, which to me seems like the only way out of the current political chaos.