Someone please explain to me how you just “lose feelings” for your partner

My ex (23M) broke up with me (22F) over a single phone call after 1 year 3 months of relationship.

The reason? “I don’t love you anymore. I lost feelings” When I asked why and if I had done something wrong, he said I didn’t and “it just happens sometimes” . Did not talk on video call even after repeatedly asking for it. He basically had zero respect for my needs or feelings. He was very cold and stoic throughout the conversation and provided no further explanation as to what led him to do this.

I supported him throughout the relationship, I loved him, cared for him. In my opinion, there was nothing better I could do. He was extremely busy these past couple of months (he’s in the defence forces) and I never even bugged him for time or attention even when he gave very little if at all. I waited for him.

I was there for him, put up with his manipulative family’s bullshit. And in the end he left me with no prior notice, complete blindsight.

It’s worth mentioning that he did something very similar to his ex too. Left her the moment things got tough for him at work and she was asking for his time.

It’s also worth mentioning that his family (mainly father) was against me for no apparent reason except to cause unnecessary drama and chaos. (I have a feeling they did the same to his ex)

How does someone just lose feelings like that while completely acting normal before? Do they ever feel even the tiniest bit of regret for hurting someone that loved them?

TL;DR : Ex left for no other reason except that he lost feelings and I can’t figure out why