As always, go back to the classical definition of probability.
Number of ways in which you can arrange 10 people in a line such that 2 of them are always together: 2*(9!) This can be understood by treating the two people who are supposed to be together as one person. You can then arrange these 9 people in 9! ways. In every permutation, the group of two people can appear in 2 ways, that is AB or BA. So, total number of ways will be 2*(9!).
Total number of ways to arrange 10 people in a line: 10!
Probability = Favourable outcomes / Total Outcomes = (2*(9!))/(10!) = 1/5