For me it's not so much cycling I get a bit bored with it's over familiarity with my local roads.
I might go out cycling maybe 4 days a week, circular routes starting/ending at home. So over the years local roads have become very familiar, even for time of year, time of week (ie Wed afternoon traffic often a lot quieter). I know exactly what is round the next corner, in fact what is round every corner and rides can become a bit "going through the motions" more about exercise than enjoying being out.
However, few years back I start riding a tadpole recumbent and that put new interest in my rides. No idea why 'cos exactly the same roads, corners, etc. These days I find a change of cycle can have a real boost in enthusiasm eg yesterday I went for a ride on my DF-upright and it was really fun (same roads).
Weird thing is I have a good bike carrier on car roof that's quick and easy to use and tadpole recumbent quickly folds and fits inside car easily but for some reason have a real mental block about putting cycle in/on car and driving somewhere different. Maybe because I regard cycling as environmentally "good" so the idea of burning petrol for a ride grates too much for me to do it - which is daft as many of my rides are recreational rather than utility (shopping, etc.)