Would you please allow me to expound with two personal examples of this issue.
1. We were learning, in Rav Abadi's Kollel, a particular subject pertaining to the Laws of Yom Tov. The Mishnah Berurah states that "under the above mentioned conditions it is permitted to do so-and-so."
So, I asked the Rav "so that means that it is Mutar (permissible)? He said "no. The conditions are too numerous to remember and from a practical stand point you'll never remember to fulfill all of them"
So, here is one example why you need a Rav, and not make, even seemingly simple dicisions, by yourself.
Now, here is the kicker.
Another question arose, and as usual, I asked Rav Abadi what to do. I would like to mention, that almost everytime I asked a question from my Rav, I made an effort to look up the shulchan aruch myself. For one, I don't want to sound completely ignorant, although I actually am, and secondly, so I can see how my Rebbe looked at the same halachah that I saw and how interpeted it.
Anyway, he gave me a psak which I can clearly say was different from what the Shulchan Oruch stated. It wasn't an issue of being more stringent or lenient. It made no difference to me. It was a matter of what was the right thing to do.
Well, much to my Rav's dismay, I kept pestering him for an explanation, and all I got was a "nothing doing" answer.
Well, needless to say, I did what my Rav told me to do, but it sat hard on my heart.
Years later, and I mean YEARS, it finally hit me. (Did I mention I'm a slow learner?)
I finally understood what the Gemarah meant when it stated "Ein Adam omed al da-as rabo..."..I them realized that my Rabbi not only took the Shulchan Aruch into account, But also me.
I know it's hard to understand how a halachah can change from person to person..But until I have permission to repeat this Psak, I can't.
I then, for the hundreth time, realized how fortunate I was to have Rav Abadi as a Posek and Rebbe. (I'm sorry Rabbi if I'm embarrassing you).