At 39, I’m starting to see more relationships fall apart than begin. But for those that are still lasting, the one constant seems to be that each partner is fairly equal in accomplishment. People with similar levels of accomplishment tend to be of similar age, income, wealth, and experience.
Among the many reasons why people break up, a lack of respect might be reason #1 followed by resentment as a close #2. Let’s say you’re much more accomplished than your partner; you will start losing respect if you catch him/her lounging instead of hustling to at least try to get to your level. If you’re the partner who is lounging, you’ll start resenting your partner for being so demanding, especially if s/he is much older than you.
The physical passion only burns for so long until substance takes over.