It's good advice to avoid cosigning for any person, family or not. But here are my reasons:

1. A person that needs a cosigner is almost invariably someone that has already screwed up their credit. If it's a young person without credit, they can usually get started with credit without cosigning, so it leaves people that for whatever reason have not been good with credit in the past. In other words there's a reason creditors are not willing to extend a line of credit or financing to them alone.

2. When you cosign you are basically taking on the responsibility of the credit being paid back. Sure you may think that the person who has the financing is responsible but they are literally riding on your score and reputation with credit. Why do I say this? Because any delinquency and esp. default falls 100% on you. You will have to get this square or suffer the credit damage. Sure the debtor takes a hit too but hey their credit already sucks so YOU are the only one to suffer if all goes bad. You might as well take out a loan yourself and lend it to them hoping they will pay it back!

in your case, this is a person that you are attached to for now but there is no permanent commitment. Anything goes bad in the relationship and if it's bad enough, they will leave you high and dry or at least can and just stop paying on loans they know you'll end up bearing the burden of. There's really no accountability attached to a loan that's cosigned. I wouldn't cosign for a family member, how much less for someone like a bf/gf that isn't necessarily going to remain?

2 years in a relationship does not make a solid ground for any financial commitments. I honestly would not do anything financially tying you to them in any way, like buying a car together, a house, or anything that requires financing at all. Heck I wouldn't even share a bank account til you're committed.

think very long and hard about the question of whether you'd be willing to take on a loan yourself of $xxxx amount in the first place, if not, cosigning for that same loan is a very very very bad idea.

some references:
5 Reasons Why Co-Signing A Loan Is Bad For Your Financial Health

Why You Should Never Co-Sign (And 6 Things to Do Instead)

Cosign for a Credit Card - Reasons Cosigning is a Bad Idea - CreditFast(R)