I'm not sure about sentiments. First I had a brother and sister who got the deeply connected sentiment after she shared stories of personal challenges with him and I thought that was amazing. Then I sent two good friends on vaycay and they did everything together. Skiing, hot pot, relaxing in the onsen and hiking in the bamboo forest. In between they became best friends and fell in love and had their first kiss and nothing. Not a single sentiment between them. So disappointing!
Edit: Feeling stupid, I was only looking at the tab on top of the screen when they were interacting, When I actually went into her sim profile in the relationship panel she had many sentiments about him. Deeply connected, brought together by hot pot meal and played together at the festival of snow. The only sentiment he has is Snowy vacation memories but he is self-absorbed so yeah