Fire and earth on the same ambition
Leo supplies presence and Capricorn supplies structure, and used well that is a strong division of labour. Leo makes the case in the room and Capricorn makes sure the case survives contact with the details afterwards.
Used badly, each disparages what the other is good at. Capricorn treats display as a substitute for work, and says so in a dry way that lands harder than intended. Leo treats endless caution as a failure of nerve. Both criticisms are recognisable, and neither is fully fair.
Cardinal and fixed
Capricorn opens and Leo holds. Capricorn will start a project, a move or a plan without much discussion; Leo will commit to a way of doing things and then decline to be redirected.
The friction concentrates on being managed. Capricorn's instinct with a problem is to take charge of it, which Leo experiences as being demoted in its own life. Leo does not need to be in charge of everything, but it does need to be treated as a principal rather than a resource, and Capricorn frequently forgets to make that distinction visible.
Saturn against the Sun
Capricorn is ruled by Saturn and Leo by the Sun. Traditional astrology places these two in something close to opposition, since the Sun is at home in Leo and Saturn governs the sign across the zodiac from it.
What that implies in practice is that the pairing sets restraint against self-expression. Saturn's contribution is limits and duration; the Sun's is warmth and confidence. Whether the result is a partner who steadies you or one who dims you depends on the actual placements rather than the rulership diagram.
Where it grinds
Praise. Leo needs appreciation stated out loud, regularly, and treats its absence as evidence of indifference. Capricorn considers stating the obvious slightly embarrassing and assumes reliability speaks for itself. Neither position is wrong and neither is understood by the other without being said explicitly.
The second grind is spending. Leo spends on the occasion and the gesture; Capricorn spends on the asset. Each finds the other's priorities faintly irresponsible, and money arguments in this combination tend to be about values rather than sums.
Beyond the two suns
A sun sign covers everyone born within the same few weeks, which makes any pairing description a generalisation across a very large population rather than a statement about two individuals.
The Moons determine whether the household is comfortable. Venus determines whether Leo's need for stated appreciation is met at all, which is the decisive question here. Mars determines how the competition for authority plays out. Read those before drawing conclusions from the suns.
Questions people ask about this
Do Capricorn and Leo compete for control?
They compete over being managed rather than over goals. Capricorn takes charge instinctively, and Leo objects to being handled rather than consulted.
Why does Leo feel unappreciated here?
Capricorn assumes reliability communicates regard. Leo needs it said aloud and does not treat quiet consistency as an equivalent.
Is Saturn against the Sun a problem?
It sets restraint against self-expression. Whether that steadies or dampens depends entirely on where those bodies sit in the two charts.
What is more useful than the sun signs?
Venus especially, then the Moons and Mars. Venus governs whether appreciation is expressed in a form the other actually registers.