Surely the “ambition for office” within the Church can have no more minor manifestation than the desire to be senior companion as a missionary … yet how many of us who served missions did NOT look forward to the day when we would be senior?
Seniority of Elders 
In the case of two or more Elders appointed to labour together, one of them is the recognised senior, that is to say he stands first – the first among equals. This is in harmony with the spirit of our Church organisation, which embodies a perfect plan of orderly assignment. To the senior Elder of any pair or group the Conference President will address such communications as apply to all of the little company; and, in turn, the senior Elder will speak and report for himself and associates on matters pertaining to all of them as a unit.
Seniority in these cases is not determined wholly by length of service in the field, neither by individual age. These conditions are factors to be taken into consideration by the Conference President in making his appointments; but other, and in some instances many other, conditions have to be taken into account. Some of our missionaries have had long experience in Church service at home; not a few of them have served in positions of responsibility as officers in Wards and Stakes; others have completed honourable missions before entering upon their present service in these lands. Such men may be better qualified for positions involving seniority than are their associates, and may even be appointed as Conference Presidents early in their period of current service. Furthermore, the personal element is to be considered in all appointments. Some can accomplish more as leaders, others as followers. Ambition for office arising from any personal desire for distinction is foreign to the spirit of the church; and to the credit of our missionaries be it said that pronounced manifestation of such personal ambition is so rare as to be negligible.
The success of the great cause to which we, as missionaries, are devoting time, talent and material possessions, is, as it should be, the great consideration; and every true missionary is willing and eager to serve where he is most needed and in the particular position to which he is appointed.
Conference Presidents should designate the senior Elder in each little group appointed to labour as companions; and such designation should be made with due consideration to experience and other qualifications, including personality and adaptability to existing conditions, and not solely or even primarily on the “missionary age” of any Elder.
In my case, finally “going senior” meant I could at last have some say in my personal life (what cheese or bread we would buy on P-Day, choices that had been categorically denied me by one senior) as well as some say in the organization of our work (it seemed ridiculous to me to schedule our tracting in an area that was as far removed as possible from any appointments we might have had, leading us to spend most of our working hours biking back and forth across our district). And when I was senior, I tried to let my juniors have the control over their own lives and the input into our work that I had not had.
What did it mean to you to be, or want to be, a senior companion? Were those roles determined solely by seniority, or did your mission president take any other factors into account, as suggested by Elder Talmage?