I would avoid coming up with astrologically-based presumptions about the best partner for you. There are many ways in which your chart can connect with someone else's and very often that synastry does not meet pre-fabricated notions of what makes 'good astrology.'As to your question of why ...
Probably because you're more interested in the solid, long-term than they are. Or underlying desires for control kick in (control of timing, general caution, etc).
Moon square Mars shows you're impulsive and restless, but with Mars/Asc in Capricorn prefer it to be by your own agenda, rather than someone else's. Sun conjunct Saturn in Scorpio also is a high indicator of wanting to be in control. Ultimately, you'll have to learn the art of compromise with give-and-take that allows you to take initiative and follow your better impulses, but allow a partner to do the same.
Neptune has more emphasis in your chart than you realize (It and Jupiter are parallel the Asc in Cap). So you're also a starry-eyed romantic who can easily see the best in those you are attracted to and probably aren't at the age to be ready to settle down with just one of them. Jupiter-Neptune indicates freedom to dream. So a lot of your romantic life is in the anticipation of what's next, in all the sparkling possibilities. That is different than being in an actual relationship with an actual person close-up day after day where you can start to see their flaws and annoyances.
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Soul Stars Astrology by The Declinations Guy
Expert birth chart rectification