When I first starting training in aikido my sensei made it very clear the importance of practising with as many different people as possible. Everybody you train with will have different body sizes, reactions, strenghts, flexibilities, pain thresholds etc & therefore the techniques will have to be slightly adjusted to suit each individual. Those students who practise with the same partner week in week out may look proficient with there prefered partner but really they are fooling themselves. Put them up against somebody a bit different or akward and they will probably look worse or just as average as everybody else.
My advise to you is if you like the style of aikido is to train there. However continue to train with everybody & not just the favorites. Train with them all, fat, thin, young, old, good, bad, men, women, hardnuts, wimps, blackbelts & novices. You will learn something about aikido from each. Ideally if the class is big enough you should be able to train with a different person on each technique throughout the whole class.
There is no rule saying that you must automatically train with the person sitting next to you. So go wandering around the mat, just be quick to grab the next person that you want to train with.