flask Set Cookie for every response

I use flask session in my app. In one of my handler I set session value and no session set in other handlers. But I found that in every response there is a http header: Set Cookie exists. Why does that happen?

app = Flask(__name__)
app.secret_key = r"A0Zr98j/3yX R~XHH!jmN'LWX/,?RT"

@app.route('/auth/login', methods=['POST'])
@crossdomain(origin='*')
def authlogin():
    user = User(username=username, registered_at=sqlnow())
    user.accounts = [Account(provider='weibo', access_token=access_token, uid=uid)]
    account = user.accounts[0]

    session['user_id'] = account.user_id
    return jsonify({
        'status': 'success',
        'data': {
            'user_id': account.user_id,
            'uid': account.uid
        }
    })

@app.route('/api/movies/<movie_type>')
def moviescoming(movie_type):
    if movie_type == 'coming':
        return getmovies(MOVIE_TYPE_PLAYING, offset, limit)
    else:
        return getmovies(MOVIE_TYPE_COMING, offset, limit)

app.run(host='0.0.0.0', debug=True)

Code shows here: https://github.com/aisensiy/dianying/blob/master/index.py