How to measure a relationship between interests of topics between two users?
You can create a sparse user-interest matrix, impute missing values with some reasonable value, and apply a similarity function. E.g., center all non-missing values on 0 and mutate each value to adjust for the user's and interest's mean. Then impute 0s for all missing data, and use a traditional similarity function like cosine.

You might also use Bayesian methods. You could model the relationship between users and interests. In this way you wouldn't have to impute missing data and it would account for confidence in inference and prediction.