Just as we need direction (teaching) in math, science, reading, morality etc, we need direction (teaching) in scripture. While most readily accept the former, most want to “go it alone” in the latter.
As to interpretations, very seldom does one interpret without guidance. Calvinist interpret scripture through the teachings of Calvin. The fundamentalist missionary sitting in my living room last night, through the teachings of Penscola Baptist Institutes anti catholic leader Peter Ruckman. Catholics through the Church and rely on 2000 years of teaching.
Your friend is missing something very obvious. St Paul wrote to the Church in Corinth teaching them about scripture, how to read it, understand it etc. He writes in chapter 1:10-18 they were keeping to what he taught them in regards to Christ, scripture etc. He takes them to task for seperating into groups. (I guess the missionary was right, protestants have been around for a long time

)
I agree it is possible for someone to stumble upon the truth or correct interpretation of scripture, just as it is possible for someone to stumble upon basic math equations solutions. The problem is how much will be in error until that point. St Paul compares this early church/parish/body to babes who are fed milk. The picture is of someone or group being lead to maturity as a parent leads a child. Paul is helping the Corinthians understand scripture and Christ and God. He did not turn them loose and say figure it out. Just like I did not give my 15 year old a book on driving, the keys to the car and show her the road and tell her to figure it out the Holy Spirit will guide you.

Her interpretation of traffic laws, codes, signs etc in this process would be “different” from the authorities view. She may argue she knows and I am wrong and maybe even the authority is wrong because the spirit revealed it to her. All she has to go on is her limited understanding, knowledge and experience. If we reject this method of drivers training, why do we or some think this is ok for theology?
Personal interpretation comes down to a simple thing. What Satan said in Gen 3:14. Since then we have wanted or believed we can be like God. If we have a problem with God, having a problem with any authority is a given. We as humans dislike being told what to do. Especially in the States, our history and culture is a rebellious culture to authority. Sometimes a good thing, but not all the time. The first protestants who came here were nonconformists or puritans who thought there way was better. Follow on protestants learned that the hard way. And catholics…ouch.
As a protestant, I have come to understand why so many discount or dislike the Pope. They want his job. As my 12 year old and I discussed recently, we all have a Pope. Either he is in Rome or he sits in our living room.
Fr. Mark