Praise be to Allah.
Cutting the nails is part of the fitrah . Prophet Muhammad (peace and blessings of Allah be upon him) said: “The fitrah is five things: circumcision , removing the pubic hair, trimming the moustache, cutting the nails, and plucking the armpit hairs .” (Reported by al-Bukhari and Muslim).
According to another hadith, ten things are part of the fitrah, including cutting the nails. Anas (may Allah be pleased with him) said: “The Messenger of Allah (peace and blessings of Allah be upon him) set a time limit for us to cut our moustaches, cut our nails, pluck our armpit hairs and shave our pubic hairs: we were not to leave it any longer than forty days.” (Reported by Ahmad, Muslim and al-Nasai; this version narrated by Ahmad).
Whoever does not cut his nails is going against the fitrah. The reason behind this is cleanliness and hygiene, because dirt can gather under the nails. It also serves the purpose of making us different from those among the non-Muslims who allow their nails to grow long, and from animals that have claws and talons. (Fatawa al-Lajnah al-Daimah, 5/173)
Another bad habit that many people have is to leave one nail long. All of this is clearly contrary to the ways of the fitrah.
We ask Allah to keep us safe and sound. Allah is the Guide to the Straight Path.
And Allah knows best.