First he/she has to say goodbye to all their friends and family and move to a country that is foreign and where probably they don't speak the language. Intense loneliness. This is pretty obvious. It can take years to feel comfortable in the newly 'adopted' country. Possibly never. Even if good friends are made with the locals, there can be a sense of not ever being able to truly share your heart.
Through the years, though, things generally settle and the missionary feels more and more comfortable. Small and large changes in the way the missionary thinks and behaves make this possible.
Then, it is time to return home for a period, short or long. They face saying goodbyes again, this time to colleagues with whom they've shared much and local friends too, many of whom will not understand why the missionary must come and go.
When they get "home" the missionary then realises how much they themselves have changed. Additionally, the people back home haven't remained static. They've married, divorced, had kids, changed jobs, houses, moved, and certainly the spot that the missionary once held in their lives is no longer vacant. The missionary finds it difficult to find a place to belong in their lives. People find it difficult to relate to this person they once knew, but now don't know as well because of lack of shared experiences and the changes which have taken place within the missionary. Loneliness!Additionally while missionaries are 'home', they tour around different churches and groups. Their lifestyle is quite unsettled and their time is often spread across many different situations and relationships. Loneliness!
Missionaries can look like they know lots of people, but again many relationships are not deep ones where their true worries and concerns can be shared. Usually they feel the pressure to tell success stories and to keep the personal stories and the failure stories to themselves. Not many stop long enough to hear beyond the surface details. Loneliness!
"If I go up to the heavens, You are there;
if I make my bed in the depths, You are there.
If I rise on the wings of the dawn,
If I settle on the far side of the sea,
Even there Your hand will guide me,
Your right hand will hold me fast."
Psalms 139:9,10