One of the biggest mistakes fitness beginners make is tying themselves to a less motivated and less fit work out partner. Giving yourself the responsibility of motivating someone is a sure way of setting yourself up for failure. Two negatives do NOT create a positive. It only creates extra work for you.
If you inspire to improve yourself get with someone just as fired up. Or even better align yourself with someone you desire to become. When you do this you will often want to do more.
In any area of your life this can be beneficial. If your desire is to be a successful salesperson, it would make sense to be mentored by someone who is already on top.
In swimming and biking, there is a technique called drafting. This is where one person follows directly behind another in the same lane. This makes it easier for the person behind because the person in front fights the wind resistance. The key to drafting is to watch and learn from the person in front of you. The same thing happens with motivation: let their motivation work for you. Look for someone who is exceptionally motivated and fit. Work out with someone who inspires you to become even better.