I love learning and will regularly do courses to pick up something completely different, or something applicable to my career.
However, I do think it's okay to allow some of the things I've learnt to fade away. Technology or previous interests become less relevant, and trying to retain everything keeps me too focused on the past, making me feel like I get worse over time, rather than keep a positive learning attitude towards the future.
Also, relearning things I've used a long time ago, because it has now become relevant again, is a lot of fun. The second time I've looked at trigonometry, I actually really enjoyed it.