Here are some links to explain why an understanding of
dog-specific psychology,
pack structure,
social hierarchy,
and leadership
are so critically important to resolving major behaviour problems.

Why Respect? (Why should he listen to me?)

A good analogy on the relevance of authority.

K9-1: Leadership is about being proactive, and utilising good technique.

Groundwork for correct pack structure, at

Academic theory vs leadership in practice.

Obvious if you're looking--Suzanne Clothier explains social dominance.

Black Belt dog training--Why should your dog listen to you?

Black Belt training, on the "alpha" role.

Building a relationship FIRST.

Natural and artificial dominance

Patricia McConnell has a good grasp of what dominance is & isn't

Complex social relationships

Leadership in action: Why NOT take candy from a baby? (Suzanne Clothier)

Leadership basics

Lessons from the Masters. (Even corrections have their place.)

Terrierman is always a good read:
Terrierman on dominance, 1 and 2.

*Social dominance is ANYTHING but "disproven".
Quotes, with references, from scientists in the field.

Some background on where the "debunked" myth came from.

Trainers are weary...

Trainers are having to state the obvious...because it hurts to see people keep fatally failing their clients' dogs.

AGAIN: The willful avoidance of learning about dogs' social structure is hurting, and even killing, dogs.

Stephen Rafe on leadership and rapport.

Exceptional course for handlers, especially with groups of dogs or as shelter workers.

More how-to's: library selection on "Alpha Boot Camp".
Workfare program for Chows
No Free Lunch training, from Pitbull Rescue Central
Claiming space
Pack leader tips from Chesapeake Bay Retriever rescue
Shirley Chong, Mind Games
Sophia Yin, Learn to Earn

Choosing a trainer: experience counts! Flexibility counts. Understanding dog-specific psychology (not merely operant conditioning, i.e. clicker-training) counts. Read their trainer's philosophy. Here's one example (brief summary)-
Here's another one. (far more detail)
This fellow gets excellent results with a full toolbox of methods.
Another promising introduction:

If you're having serious behaviour problems, you're looking for someone with a wide range of knowledge, and experience that shows. If you can't find an appropriate trainer locally, many will do telephone sessions. For example, this lady can get you back on your feet quickly...and for the do-it-yourselfers, this course will be a big help. (Yeah, it's heavily marketed :-/ but covers a lot of important stuff, mostly in video format.)

A broad understanding of how dogs think is the underlying foundation for fast and effective rehabilitation. If you haven't tried a wholistic method, meeting the dog partway & establishing two-way communication, then you haven't "tried everything"--in fact, you are missing the biggest part of the picture. Secure, submissive dogs don't resource-guard, and in fact they rarely take it upon themselves to aggress at all.
Sometimes love (and cookies) are not enough--many dogs need the security and guidance of pack structure to function in our society, and providing that structure resolves most major behaviour problems in one swoop. Hopefully the info above will give you the tools you need to re-shape your relationship, and/or help you to locate a skillful, balanced trainer who can walk you through your dog's difficult behaviour.