maybe if you have to think about a sequel it's not meant to be, eh? i honestly think some people are more in tune with sequelization than others. it's not good or bad. i've thought about this before, and there's not a single book i've written that i couldn't derive a sequel out of, eventhough i tend to kill off every single character. especially when there are years separating the original book with the idea of going back into that universe, you can look back on the thing with fresh eyes and a different perspective and be surprised at how many avenues you have to go down when you used to think you tidied everything up to a tee.
also, people are mostly suggesting sequels as a continuation of the original plot as opposed to sequels being episodes, one not connected to another by anything other than characters and setting. detective novels do this (that's probably one reason i'm not into them), as opposed to there not being a large amount of sequels for romance.
my opinion is that if you have to consciously insert potential sequal material into a book, maybe that universe isn't rich enough to sustain a sequel. if you do these insertions you run the risk of sounding obvious, and when you do that i think you start sounding to the reader like you're in it for the money. my personal opinion is sequels should be organic, but if you want to do a sequel, i suggest having a vague idea of what that sequel would be about so you can plant some very, very small seeds now. it's impossible to make direct suggestions without any information, of course, but for an example of what i think an 'organic' sequel might entail would be how the father always went on business trips to a foreign country, so that when the girl finds her dad murdered and she resolves that book with the people who killed him, there's still a little fodder left for the sequel, like one of the dad's other trips lands him business associates who then comes and wants something from the girl. i don't know, something like that, lol.