Eurus wanted him to face his feelings 

 (Sherlock meta by cherish--these--times)

Don’t tell me that someone who presumably convinced Jim Moriarty, in the spam of five minutes, to kill himself if necessary and predicted three terrorist attacks on twitter in less than an hour did not foresee Sherlock having to say 'I love you' first. That was the whole point. Eurus wanted him to face his feelings and prove to him how detrimental they are to his thought process. The prospect of losing Molly Hooper made him lose his cool and his logic. (Duh, of course there was no bomb in her flat you dummy).

He knew that Molly loved him. He knew that asking her to say the words was cruel. And that’s why the whole make-her-say-it procedure took far longer than it should have.. He hesitated, rambled, begged. He was a mess of emotions. A disconnected Sherlock would have lied to her face in order to save her life 10 seconds into the conversation. This is what he is best at. Seeing the bigger picture and putting feelings aside to achieve the goal. Being a soldier. And he is brilliant at it. But this is the woman he loves. Hurting her hurts him too. That’s how it works. Add a death threat on top of it that turned out to be a scam and that’s what you get.