WHY DOES THIS HAPPEN?
Spindly, leggy growth can have a number of different causes, most commonly including an imbalance in the amount of fertilizer that your plant is receiving and / or unsuitable light conditions. This, in turn, can lead to further health problems with your plants.
For instance, if your plant is kept somewhere that is too dark, it will stretch itself out to try and reach any available light source nearby, leaving it looking sparse. Additionally, if your plant isn't getting enough light then it won't have sufficient energy to grow healthy foliage, which is why it can start to look leggy.
However, some plant types may actually become spindly regardless of their health. For instance, plants that die back in the winter can become spindly when they experience a growth spurt in springtime.
While spindliness isn't always necessarily a problem, it can affect the appearance of your plant and, if left unchecked, could eventually cause your plant to become top-heavy and at risk of toppling over. If this is the case, it's probably a good idea to repot your plant.
Your plant's stem becomes longer and thinner than usual
Its stem may look curved/ start to bend
The thinned stem may break under the weight of leaves or buds
Note that vining plants may also appear spindly if they're not given something to climb on. This is easily fixed - just provide your vine with a trellis or other suitable surface that it can climb on and it will start to look healthier and fuller in no time!