The Ice King wants to have someone to love,he's on the talking living creature in the Ice Kingdom,so he wants someone to keep him company in the lonely Ice Kingdom,and considering he's a King that's probably the reason why he's going after Queens.
I think that Ice King/Simon Petricov is interested in PB because she resembles Betty.I think some where deep down Ice King wants to return to his old life,and is trying to fight the crowns power.
Pb has the same color skin and has the intellegence of Betty,making Ice King attracted to her.Because somewhere deep down he's still Simon.