You have to neglect the negative information of your current situation and must focus on the future opportunities by staying positive. 

You can say for example 

1.    I want to explore more opportunities and the job vacancy you are offering is the best platform to do much into it. 

2.     I like to work on challenging projects and I also want to share my ideas to improve the process in best possible ways. 

3.    I am keen on getting familiar with, and this job gives a chance to use my present subject matters and increment my abilities 

4.    I appreciate filling in as a major aspect of a group and am searching for a chance to deal with a fascinating undertaking. This activity is a piece of a group chipping away at an intriguing venture, and I would love to participate in this work. 

Your attention here is on (something that you like), and I truly appreciate doing (whatever that is). Along these lines, I hope to expand my pleasure in my work when I am ready to concentrate more on (that part of the activity). 

You can exhibit your enthusiasm for this new boss by saying: 

1.    I've appreciated working with an extraordinary gathering of individuals at my present boss, yet this open door in this organization fits great with the heading I need to take my profession. 

2.    I've taken in a lot in my present place of employment; however I'm keen on working at [name of employer] dependent on the incredible things I have found out about this association.