A relevant hadith is:
Narrated 'Abdullah: that the Prophet cursed the women who practice tattooing and those who seek to be tattooed, the women who remove hair from their faces seeking beautification by changing the creation of Allah.
Jami` at-Tirmidhi [grade: sahih]
There's other comparable ahadith: Sahih al-Bukhari 5931, 5943, and 5948, Sahih Muslim 2125 a, Sunan Ibn Majah, Sunan Abi Dawud 4169, and Riyad as-Salihin.
Generally, these ahadith express that it's forbidden because it's changing the creation of Allah for the purpose of beautification.
There's forms of modifying one's body which are encouraged, e.g.:
Abu Huraira reported: Five are the acts of fitra: circumcision, removing the pubes, clipping the moustache, cutting the nails, plucking the hair under the armpits.
Sahih Muslim 257b
Narrated `Aisha: I applied perfume to the Prophet with my own hands when he wanted to assume the state of Ihram ...
Sahih al-Bukhari 5922
And in Islam, beauty is not ordinarily a bad thing:
... Allah is Beautiful, He loves beauty ...
Riyad as-Salihin (see also SunnahOnline.com)
So why plucking eyebrows is singled out is not clear. Islam Q&A summarizes it as follows:
It should be understood that with regard to changing the creation of Allah, some of it is permissible and allowed, such as circumcision, shaving the pubic hair, plucking the armpit hair, and removing defects and deformities.
[They quote Satan in Qur'an 4:119 (I will command them so they will change the creation of Allah) to emphasise the importance of not changing the creation of Allah.]
There's a lengthy discussion on this topic in a daruliftaa.com fatwa. There are diverse scholarly viewpoints as when this prohibition applies:
- For Hanafi fiqh, Tahtawi is quoted as saying
when it is done without the permission of the husband
(likewise for Shafi'i), and Ibn Abidin is quoted as saying when
it is done for the sake of beautifying herself for strangers
when it is done without a need.
- For Maliki fiqh, the fatwa quotes the book Hashiyat al-Adawi which says
for a woman who is forbidden from adorning herself such as a woman whose husband has passed away.
- For Shafi'i fiqh, the fatwa also quotes Khatib al-Shirbini saying
for the prohibition is deception – such as an unmarried lady deceiving a prospective husband.
And the fatwa writes "the most reliable position in the Hanbali School is that there is no exception", but there were Hanbali scholars who thought there were exceptions.
The daruliftaa.com fatwa generally concluded:
Shari’ah allows bringing back to normality areas of the body that are deformed, damaged or considered abnormal by ‘sound’ people (and not what the fashion industry dictates) – especially when it results in hardship and psychological pain, and more so, when it may affect one’s aspirations to marry.