It's normal, for people, that the mind drifts into pleasant and also into unpleasant thoughts. What's Rosemary's story? I think she gives it in the Satipattana Sutta talk, about the mind drifting with how beautiful the trees look and this and that, and then remembering India, and then remembering an old boyfriend who broke up with her. All of a sudden the mind is thinking about an unpleasant thought even though it started out pleasant. That's one of the dangers of letting the mind drift.
When it comes to reminiscing, remembering pleasant things from the past, if your mind is normal, sure, go ahead, remember some of those nice things occasionally, and then watch what happens with your mind as it keeps remembering, moving from one subject to another - as Rosemary's example goes - starting with a pleasant subject, and it invariably ends up in an unpleasant one.
Would you like to be more in control of your thoughts - instead of your thoughts being in control of you? It's something to think about whenever you are daydreaming, or having wandering thoughts, at any time during the day. Would you like to be more in control of your own thoughts?