You can be head-over-heels for someone because of their presence, what they give off energetically. You’d like to try out their energy, even if it’s different to where you currently are.
You desire to experience (marinade in) that new vibration and then decide if you want to harmonize with it. You seek to find out for yourself what frequency fits you.
When opposites attract, each individual is intrigued by traits they do not possess. (Why were you attracted to the bad guy, or the party girl or just the wrong one?) You look to find out what you want to incorporate in your life.
Yet, to be in harmony with each other in the long-term, the two of you must equilibrate and in time be in harmonic presence (vibrate at a similar frequency).
Opposites might attract, but likes stay connected to likes.
The same holds true when two individuals may meet while they are on the same vibrational level. However if they do not grow at a similar rate over time—or they grow in different directions—they are no longer attracting, but repelling.
Here’s an interesting phenomenon in nature: Water will always seek it’s own level. Incidentally, the average person is about 65% water!