With most (new) relationships there is an "infatuation phase".
This is a period where both people tend to bend over backwards to impress one another. The word "no" is seldom if ever used as no one wants to risk "blowing it" with the (new) object of their affection.
Conversations flow naturally, laughter comes easily, making sure the other person is happy is a top priority, token gifts, cards, spontaneous dates, and getaways have them floating on air in addition to the passionate sex!
A naïve or inexperienced dater often mistakes the "infatuation phase" to mean they've found their "soul-mate" or believe they're "in love".
This phase in a typical relationship generally lasts 3-6 months or possibly up to year. Things usually change after the first major argument/disagreement.
This is when both people start to reveal their "authentic selves". You get to see what their "expectations" "boundaries " and "deal breakers"
You also get to witness how they handle stress and disappointment.
In other instances one feels like a victim of "bait & switch".
Instead of "unconsciously" going through the "infatuation phase" they believe their mate (deliberately) set out to win them over with the intention of either dumping them or it's their basic "modus operandi" to RELAX once there is an emotional investment/commitment.
Some people believe "commitment" means one gets to gradually STOP doing all the things that caused you to fall in love and you won't leave.
These people can't wait to "be done" with the dating/courtship stuff!
In other instances some folks experience an "aha moment" or some realization that they aren't being (true to themselves) when they're with this person. They've been "sleep walking" through the relationship because it was comfortable.
Having said that when it comes to losing interest in a "casual dating" scenario there could be a variety of reasons why one loses interest.
Sometimes (you discover) they lack too many of your "must haves", they have a habit you can't stand, there is an "incompatibility" of some kind, a glimpse into their personality or expectations are not in line with what you want, or there is something about "their world" that keeps you from being able to imagine yourself hanging in there with them long-term.
Last but not least not everyone approaches dating for the purpose of finding a "soul-mate", marriage, or looking for a "long-term" exclusive relationship. Depending on where one is in their life they may just be dating for fun!