Hi yui,
The other requirement is to have flirted with them in their bond conversations (the flirt options were marked) depending on the character flirting 3+ to be safe starting in bond convos 4 and 5, otherwise most of them will reject romance because they will find it too strange and vice versa ( if you flirted many times but try to go friendship route, they will be disappointed and heart broken) there's a few exceptions ( Samuel will go friendship or romance regardless of how many flirting ) but to be on the safe side if you want to go romance flirt Everytime the option is available if you want to go friendship, avoid flirting.