It's a very fun game. It's cool to play with friends. I didn't really enjoy Warframe until I hit level 30 on my frame and weapons. You can enhance your gear with mods to make you stronger.
About fishing, yes, you should level up. The mobs in the area will land from time to time and annoy you. It's always nice to deal with them quickly and get back to the fun stuff.
What kept me playing was setting goals. It kept me interested. I also like tracking my progress. There's certain modes I hate playing (e.g. Archwing missions). I just don't play those and it works out for me.
Oh yes, setting goals usually helps to keep me interested in a game aswell. For now though, the only goal I can set for Warframe is to level up and learn more about the game :-)