Aeris is the last Cetra (ancient human race that could speak with the planet.) She was not alive when Cloud made his 'Reasons for Fighting' speech, and therefore had no opinion on it. Aeris loves flowers, and grows them in her church,
(In the Sector 7 church)
Aeris: Don't fight on the flowers!
She learned a lot at Cosmo Canyon. Aeris has a better understanding of the Lifestream, and the planet's history than Barret, as shown here:
(The entire party discusses what's going on at the Golden saucer)
Barret: Cetra? That some kinda disease?
Aeris: That's what the Ancients call themselves. Hey, didn't you listen to the elders at Cosmo Canyon?
In "Maiden who Travels the Planet", Aeris meets the ghosts of Biggs, Wedge, and Jessie, and they are remorseful about civilians dying in the bombing of mako reactors. Aeris compares this to her own remorse, as she sold flowers to make a living, but this killed some flowers. This shows that she has compassion for all life, and does not make distinctions based on species. Aeris later dies casting the spell "Holy", but before she can complete it, she is stabbed by Sephiroth, who is trying the bring about the apocylapse by summoning a meteor. Aeris' sacrifice was done in part to protect the planet, being the last Cetra.
(From Maiden Who Travels the Planet) She [Aeris] even knew about the world that a Cetra would reach in the end once they had fulfilled the mission they had on the Planet. That was why she accepted death fearlessly even when she had a strong feeling that it was going to happen to her soon one day.
