Rabbit seems to hate going outdoors

Hi all,

Now the weather has improved I've started taking my rabbit into my back garden for her to enjoy - it's basically just grass and dandelions. I give her water, some shelter from the sun, and a whole lawn to eat, and although she does scamper around eating grass and dandelions, she keeps wanting to get back inside. If I leave the door open, she'll go straight back in upstairs (she has claimed the attic as her own), and if I try to block her getting in, she'll really try to force her way in. Just wondering why she's not more keen on this newfound freedom!