Are you:
a dream? (1. dream as in the sleeping subconscious state or 2. dream as in future aspirations)
I'm easy to start
1. they start on their own usually, 2. it's easy and encouraged to have dream goals
But hard to finish
1. I've personally never "completed" a dream, 2. achieving ones dream goals is never easy and usually takes hard work
You might spend time with me at night
1. dream when you sleep, 2. sometimes you make plans or work towards your goals at night (after work)
Or while you're at work
1. day dreaming (awesome past time), 2. working towards your goals professionally
Sometimes simple things turn into me
And that is the worst
1. dreams about mundane things could be annoying? 2. sometimes seemingly simple things are the hardest to achieve: like dieting lol.
hint: Sometimes it's me and one person, but most notable examples of me include a group
usually one person dreams, but often times, dreams involve other people (same with achieving goals etc)