Lead owner management

Dear community, 

I'm working with lead and I discovered that when I change the lead to "partner sale" and I choose the partner it always reset the lead owner. 

How can I avoid this behaviour? 

I don't want the owner to be clear every time I change the partner. 

I looked in the business rules but there was no business rule to make the owner related to the partner account. 

Can you help me to find where in the code of the page I have to change this rule?

Thank you in advance. 

Best regards,

Clara