I know this question gets asked a lot but, I've yet to find an answer which really gives me an explanation. My understanding of korean grammar is a bit basic but here's what I know:
보고싶다 = Dictionary form/plain, or diary form, however you want to call it.
보고싶어 = Conjugated to present tense and is not formal/polite.
I also know basic past and future tense conjugation for verbs and adjectives of varying politeness.
So, knowing that, I'm confused about why someone would say 보고싶다, putting politeness aside. My boyfriend says it to me, whereas I say 보고싶어.
I would really appreciate it if someone could provide me with some clarification!