Currently, I am employed as an intern at a company where I am doing the work for my master thesis. The internship was initially limited to six months. As it turns out the thesis will not be finished by the end of those six months. Therefore, my boss suggested an extension of my contract for another six months.
I realize an extension makes sense, but I am not capable of staying that long.
The work itself is interesting and rewarding but the surrounding circumstances are becoming increasingly unbearable for me. For this reason and the fact that I already made (personal, non-professional) plans for the second half of the year I can extend for three months maximum.
During this time I can finish the thesis work but might not be able to finish some other (tedious, time consuming) work that is not directly part of the thesis.
Is it acceptable/professional to tell my boss I can only extend for three months (for mainly personal reasons) as opposed to the six months he deems necessary?
Am I in some way obliged to stay for as long as my boss thinks that he will need me?
If asked, do I have to disclose why I cannot stay longer?