It’s a tradeoff between comfort and security since the fact that you control a domain now doesn’t guarantee you’ll be controlling it in 5 minutes, not to mention 3 months. This is why Let’s Encrypt gives you tools to automate the renewal process. I also recall them talking about gradually lowering the certificate lifetime so you’d have no choice but to use automatic renewal.

Relevant link: https://letsencrypt.org/2015/11/09/why-90-days.html