Verkehr mit Menschen verführt zur Selbstbeobachtung.
Association with human beings lures one into self-observation. [Kaiser/Wilkins]
Dealings with people bring about self-scrutiny. [Hofmann]
If I'm not nuts, this one could be translated: intercourse with people seduces one into self-observation (with a distant idea of masturbation behind it?). Even if this is too much of a reach, there is something similar, the relationships, in the original. What I do with others, I learn to do to myself. This kind of reversal onto oneself happens all the time in Kafka. The accuser, especially, becomes the accused just like that.
The simplest explanation would be that I watch myself so as to avoid looking as bad as the person next to me. Every new set of social circumstances remeasures me with its own yardstick. I also have to consider the effects that my actions will cause them to feel.