Learn English – ny term about morning laziness

Consider eavesdrop (“To hear a conversation one is not intended to hear; to listen in”) and overhear (“To hear something that wasn't meant for one's ears”).

A usage note at the first link points out:

To eavesdrop usually implies the listener is purposefully trying to hear the conversation of others. To overhear is more often accidental.

There are may synonyms out there for obsession in various forms - some can be perfectly fine in one scenario but carry a negative connotation in another, particularly when it comes to personal relationships.

For example, it it perfectly acceptable to be infatuated with a new hobby, but describing oneself as infatuated with a new girlfriend can come across quite badly.

One expression that comes to mind that covers all of the above scenarios without carrying a negative or overbearing connotation is 'really into'. I'm not sure if it fits into your idea of 'original and good-humoured', but consider the following substitutes to your original sentences.

1. Let's say you have a friend who is really into cats...
2. Another girlfriend is really into white-haired men...
3. A male friend is really into women who are petite...
4. Another male friend who is really into skiing...