Why would a priest want to celebrate Mass ad orientem?
A priest may desire to celebrate Mass ad orientem because it allows him to focus all his attention on the sacrifice of the Mass. When people face one another it is natural to think of all sorts of things and, if one has any empathy, react to the people they are seeing. This is a natural and good reaction. But, it is also counterproductive to remaining focused on the prayers and actions of the Mass. When facing the people, some priests may be tempted to add additional inflection to their voices in hopes of attracting bored attentions to the Liturgy. At the same time, this attention to the people takes away attention from the prayers themselves. It is also not rare that disruptions sometimes occur in Mass such as someone fainting or moving around the sanctuary inappropriately. By facing in the common direction, the priest is free from feeling the need to perform, avoids distractions and can simply pray to God. Likewise, should the priest lack reverence at the altar or is tired, the people avoid seeing the expression on his face and being drawn from the prayer.