the scene was just not properly written...when rishi finds out that swetha is ready to be surrogate mother , he should obviously be thankful (which he was shown that he was in the rose scene), but the anger and the harsh words came from the doubt that he has that swetha hid the truth from him inorder to marry him...that is why he is suspicious and even when he is grateful he wants to make it clear to swetha that by being the surrogate she is doiing him the biggest favour on earth which he will always appreciate but that will not make him fall in love with her or marry her...
sentiments are correct , timing and way to say it is wrong...and bound to hurt sweta