Tony was a very observant person. He wouldn't have been much of a genius if he wasn't. An interesting case study for his mind was the relationship between Clint Barton and Natasha Romanov.
Now no matter how much both deny, their relationship went deeper than friendship.
Contrary to what most people think Tony had met Clint before. The Archer was on the rescue team sent to Afghanistan.
Tony had liked his humour and made the man new hearing aids.
He met Natasha as Natalie and found her interesting because Tony had immediately saw the mask the spy had been projecting.
Also her reaction to Clint being taken proves Tony's theory.
Another is Natasha's jealousy when a girl pays attention to Clint. Tony likes to provoke Natasha since she doesn't like showing emotions, but they are seen where Clint is concerned.
Also Clint has a fear of screwing up his only stable and good relationship.
Tony is not going to give up trying to get them to admit their feelings.
Their lives are too short and jobs to dangerous.
They both deserve to be happy.
And Tony will see to it as he cares about his friends.
Clintasha will happen.
Tony will make it happen.