Please help

This answer contains a lot of SPOILERS about the plot, who hasn’t read the story so far.
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So basically it’s a mixture.

First there was the trigger that Yuki bit an innocent omega cause he started a rut. So En wanted to protect him not living through another trauma (the first was his parents death). As Yuki forgot about the whole incident and thought it was En who got bitten he spontaneously reacted to protect him, instinctively.

First he just wanted to do it as long, until Yuki got his memories back and the incident wouldn’t hit him as hard (youthful thinking at this point).
But it didn’t change, he wanted to end the relationship with another lie to undo the pair. But Yuki declined as he loved En.

The whole lie went on and on to a point where En couldn’t tell the truth anymore out of fear to break Yuki.

And as basement of that you have En subconsciously longing for Yuki. He basically loved him from a very early point on (that’s why he lied about his test results to begin with) and wanted to be his omega from the start, wanted to be loved, didn’t want to lose him and the very reason why he wanted to protect Yuki.

Oooh ok, thank you so much for the explanation!