They didn't really need to say I love you. They showed eachother with
other words/actions and they understood that they loved eachother.
Enough said.

Well, Kovu DID say that he loved Kiara...just not to her
face.He was talking to himself before simba talks to him,
Kovu says,"Kiara, zira, and a plot.I was part of it, but I
don't wanna be,...It's, because I love you..."

Pft! It wouldn't be like that because they are lions, they
would Nuzzle and Lick to show their feelings!

Yeah, I agree. Dialogue is neccessary sometimes, but it's
often much more intimate (and I don't mean that in a
physical way) when something is just understod simply by the
bond two people (or animals in this case) share with each
other. And then there are cases like what K&K pointed out
about Kovu going back to the pridelands after the ambush,
knowing that everyone would think he was guilty, or the
example following that about Kiara's faith in Kovu. Actions
speak louder than words.

Right, they don't say it. Yet, both couples are completly
sure of the love they have for each other.

And
THAT'S EXACTLY WHAT MAKES IT SOO BEAUTIFUL, CUTE AND
ROMANTIC. Besides, there are several ways for
saying "I love you" without pronuncing these words. Remember
these scenes:

-Simba and Nala fighting, and how
happy they are when they recognize each other.
-Nala
huging Simba saying "I missed you" and Simba huging her
back.
- "Can you feel the love tonight?", Nala licking
Simba and the way they stare at each other.
-Kiara &
Kovu laughing and purring when they accidentally kiss for
first time.
-Kiara nuzzling Kovu when he says "May be
there's a darkness in me too"
-"Upendi", when Kovu
can't swallow a passionfruit and Kiara kisses him, look at
Kovu's face.
-Just after "Upendi", the way Kiara and
Kovu say "Good night"(it really sounds like a 'I love you' )
-Kovu going back
to the pridelands after the ambush, knowing that everyone
would think he was guilty.
-Kiara leaving priderock
and believing in Kovu's innocence.
-"Love will Find a
Way", when Kiara and Kovu see each other ("...and if only they could feel it too. The HAPPINESS I feel with you...").

See? A smile, a look, a hug, a kiss,... they say more
than words

The only thing I can think of is that maybe Disney thought
having The characters say they loved each other would
Humanize Them to much.