The meeting of two personalities is like the contact of two chemical substances: if there is any reaction, both are transformed.

This quote means that when two people meet, it can be like the combination of two chemicals, where each one is affected by the meeting. If a connection forms between the two people, then both of them will be transformed in some way – they may both change, or develop new perspectives. In other words, meeting someone new is an opportunity to learn and grow, and it can have a lasting impact on both individuals involved.