ive been thinking about data finding out about lore and being so happy to have family only to have that ripped away from him by actual, genuine betrayal. by choice. lore made the choice to try to steal data's life and hurt data's friends, which sure one could argue is not entirely lore's fault, but the point here is that data didn't lose lore by some natural event or death or an event that couldnt be prevented.
and thats really sad? data never fit in. always so different, both in appearance and the way he experiences the universe, and lore was his first and possibly only opportunity to have someone to share that with. lore, in data's mind, was the only person in existence capable of understanding him fundamentally at both the most simple and most complex levels -- only for this to be ruined in an instant, despite his optimism and hope for lore.
then lal, his second chance to have that connection and his only (perceived) chance to share everything he's learned and seen with someone who would understand, also destroyed but this time by the very fact that she felt. which is, in some ways, more awful than chosen betrayal. because her coming to life is what made it happen, her neural net failing, and it couldn't have been prevented. i think data will always blame himself for her death regardless, though. blame himself for not being "thorough" enough even though we all know this poor android spent all his time researching every possible piece of the puzzle he could and he tested her brain so many times to ensure it would work before moving forward. it isn't his fault, but he thinks it is. and he has to live with that, knowing that he brought lal into existence and he loved her and she loved him... just for her to die.