Is a young man's feeling of fear and introversion towards people, and avoidance of others for fear of their opinion, considered a deficiency in faith? And is there a solution to this problem?

Faith decreases by abandoning obligations and committing prohibitions. Your introversion and fear of people do not harm you as long as you fulfill Allah's rights and observe His sanctities. However, if fear prevents you from performing duties like congregational prayer or enjoining good and forbidding evil, then you are sinful. You must seek the causes of this fear and treat it, knowing that benefit and harm are solely in the hands of Allah, and Allah's pleasure takes precedence over people's pleasure. The believer is honored by Allah, and the effects of this honor should be evident in him by speaking the truth, advising, and condemning when Allah's sanctities are violated. Furthermore, the believer is commanded to mix with people and be patient with their harm to convey his religion: "The believer who mixes with people and is patient with their harm is better than the believer who does not mix with people and is not patient with their harm." Seek righteous company that can help you overcome these fears, and it may be beneficial to consult a psychiatrist.