I'm not sure if this belongs in the spoilers section,sorry if not.
I have Dogmeat and I read in another thread stuff about companions.
In the thread it mentioned thad Dogmeat was great at (X) level ect..ect.
I was wondering if Dogmeat was leveling up with me,or will he level up according to his own experience?

The reason I am asking is...well.. ..I'm a huge dog lover and I just couldn't stand to see him die.
Yea I know,.big softy me....So I leave him home about 50% of the time.

if you have broken steel, he is almost unkillable, at least he is with me. if you don't, then he will die at some point. i am not sure if he will level with you, even if he does, it won't prevent him from dying.

Dogmeat does level, I think. If you have Broken Steel installed then you can always get Dogmeat's puppy, should he ever die. Dogmeat's puppy has a huge amount of health but will still come out second best in a one on one with an albino radscorpion.

I usually left Dogmeat at home once I reached the higher levels. I'd hate to see him die too, but mainly he tends to blow your cover too soon and go off chasing enemies. I spent way too much time chasing after him trying to keep him alive.

Thanks for your replys.
I have GOTY so I guess if I loose him his puppy is an option if I don't want to load a previous save.
Sometimes when speaking to him I have the option to give him a stempak,so I give him one.Why is this?

You can manually give him a stimpack if his health is below 100%. Otherwise all companions will reheal completely after any battle.

I guess this is a dumb question, how did his health get low?
I mean...he was in my house chillen out and out of the blue I notice his health is low.

Its not a dumb question at all and it can be caused by a few things that are worth remembering. Any injuries they sustain outside of combat is not rehealed until you fix them with a stimpack, or engage in your next battle (so an easy way to do it is pick a fight with something weak, like a mole rat). It can happen by walking over a mine, bear/shotgun trap etc or being caught in part of a bigger explosion, like an exploding car.

Also, if you were in a fight, but ran away instead of finishing it, then any companion would not be rehealed in that case, since your enemy was not defeated. This can also sometimes happen when encountering multiple enemies at once and you miss the last one.