One does not simply say I think I'll run for president today.
I think Romney's failure and Obama's reelection was the proverbial "last straw" that finally convinced Trump to run, and that he's been preparing at least since then.
Evidence: the filing date of Trump's first US trademark application for "MAKE AMERICA GREAT AGAIN" is November 19, 2012--just 13 days after the reelection of Obama.
Check it out for yourself here.