(In this one the who you are related to preference does not apply!!!)
Hodge: Magnus; Because he cannot leave the institute he thinks you will leave him for someone who can offer you everything. A magical life with whatever your heart desires.
Jace: Simon; Even though he hates the mundane he thinks that since Simon has more people skills then you would leave him for someone more funny.
Isabelle: Alec;She thinks her brother would charm his way into your heart.
Alec: Clary;He thinks Clary will take you away from him like she did with Jace.
Clary: Isabel;Isabel has way more fun and Clary thinks you would leave her for someone who could offer you more fun.
Simon: Jace; Thinks you will leave him for someone more stronger and a little more smarter.
Magnus: Jace; Even though Magnus likes to party and be wild. He sometimes fears you would leave him for someone who enjoys more adventure and takes more risk.