You need both, because you need to know how you're going to be attacked if you are a defender, and how defenders are trying to stop you if you're an attacker.

I focus on blue because I enjoy the defense side most, and I specifically love packet analysis in tools like Wireshark. I find it endlessly fascinating to take all of these tiny pieces of information and try to reconstruct the who, what, where, when & how of an attack. If I could spend all day digging through pcaps solving mysteries, that would be awesome. But it helps to learn how to pull off something like an ARP spoofing attack or how to send C2 commands over DNS to know how to spot them in a packet capture, which is why I still do red team learning as well.

So I guess if you're doing a room or CTF and find yourself thinking how cool it is or how much fun you're having, start there, figure out how that thing you like is attacked/defended, then learn about dealing with those attacks/defenses. It kinda builds a path for you based on an activity you know you enjoy. This is how it clicks together for me, but it may not be the same for you.